Length of Year
I begin with the assertion that the length of one Vulcan year is 203 days. Having looked around, I believe this number comes from the habitable zone where a planet could hypothetically survive around the star 40 Eridani A, or the real-life Vulcan. Other numbers are given in other places, but I will stick to 203 days.
Do I mean 203 Vulcan days or 203 Earth days? I need to make sure of the equivalency. Consulting Memory Alpha, it was asserted that it is not believed to be stated in canon. However, Memory Beta says it's 24 hours long. Now, an hour could also be a different measurement, I think, but we'll just say for the purpose of this discussion that one Vulcan year is 203 days long -- meaning both Vulcan days and Earth days equivalently.
From Memory Alpha, I found that Vulcans have a life span of about 200 years. (ENT: "Broken Bow"; TNG: "Sarek"). I believe that humans, at this time, have a life span of about 100 years -- more than it is now, but not ridiculously greater. Now, do they mean to say that a Vulcan has a life span of 200 Earth years or 200 Vulcan years?
200 Earth years = 360 Vulcan years
200 Vulcan years = 111 Earth years
If it were 200 Vulcan years, it would mean that Vulcans live to be the same age as humans. It is my understanding that they live longer than humans, therefore I think what is meant is 200 Earth years or 360 Vulcan years.
Age of Sexual Maturity
Next we look at the age of maturity for a Vulcan male. On Memory Beta I found this: “Most Vulcan males experience pon farr for the first time at the age of twenty, and in seven-year cycles following that, but amongst hybrids, their biological differences mean that it can come at more uncertain times.” (TOS novel: Vulcan's Glory). I've read elsewhere (though unfortunately cannot pinpoint a source) that the age of sexual maturity for a Vulcan is 35. Memory Alpha also states Vulcans canonically experience their first pon farrs at the following ages: “Spock in his 30s, Vorik in his 20s, Tuvok at age 40”.
I think it makes sense that the average age of sexual maturity in a Vulcan is 35 Vulcan years, equivalent to 19 human years. The opposite, if it's 35 human years, would be as late as 63 Vulcan years.
Or, think of it this way: The average human sexual maturity is reached at 13 years of age. If a human reaches sexual maturity (aka able to produce a baby) at 13 years, and lives to 100 years, they reach maturity at .13 of their life span. Then .13*360 (Vulcan life span in Vulcan years) would be 46.8 Vulcan years. We either mean 35 Vulcan years or 63 Vulcan years, and 35 is closer to 46.8. [I think it's logical to compare the Vulcan and human lifespan trajectories in this way because they're both humanoid, and Star Trek speaks about panspermia at some point.]
Therefore, 35 Vulcan years, but 19 human years is what I will go with.
Length of Pon Farr Cycle
The final question, then, is that of the length of a pon farr cycle. We know that it's given as “every 7 years”. But when Spock says this in Amok Time, does he mean that is 7 Vulcan years, and because he was in pon farr and speaking of a Vulcan concept, he left the unit in terms of Vulcan years? Or did he change it to Earth years for Kirk's benefit?
One argument could be that the writers simply meant in Earth years because they weren't really thinking about it much, and they hadn't yet established a year length for Vulcan.
However, I am thinking about it now, and I want something that makes the most sense for me.
If it's every 7 Vulcan years, that means it's every 4 Earth years. If Vulcans do not experience any sort of menopause, and if our age of maturity and life spans are used from before, that means that a Vulcan could experience this pon farr 46 times in his life.
However, if it's every 7 Earth years, then it's every 13 Vulcan years, and a Vulcan can experience his pon farr 26 times in his life.
One thing is for sure -- Vulcans must have contraception to use during pon farr or they'd definitely have an over-population problem. Even if we went with the shorter hypothetical cycle, that's something like 20 kids in a life-time. Even if we factored in some sort of menopause later in life, that's still going to be a huge number of kids. Therefore, they must practice contraception. As for whether they can have children outside of pon farr, it's anyone's guess.
I tried looking up the mating cycles for several Earth mammals that have said cycles, and it seems like they have their cycles quite frequently. Pandas, even though they reproduce infrequently, have a cycle every spring -- just for a very short time. Cats can have 3 litters a year, which is particularly relevant if you go with Vulcans having feline ancestors as I've seen in some fics. Elephants have a long-ish cycle between one and two years in length. So it really doesn't make much sense that Vulcans mate so infrequently.
Keeping this in mind, I would think Vulcans would mate the higher number of times to be in keeping with other mammals we know of -- thus it would be every 7 Vulcan years, and every 4 human years.
If Spock mates during the 2nd year of the 5 year mission, then it would be in the year following the mission that he'd be due to go into his next pon farr. Is this the time frame that we see him in Gol? Is that why he went? It's interesting to contemplate.
Earth Year: 365 days
Vulcan Year: 203 days
Vulcan Life Span: 200 Earth years; 360 Vulcan years
Human Life Span: 100 Earth years; 180 Vulcan years
Vulcan Sexual Maturity: 19 Earth years; 35 Vulcan years
Human Sexual Maturity: 13 Earth years; 23 Vulcan years
Length of Pon Farr Cycle: 4 Earth years; 7 Vulcan years