I think the common recommendation is at least 12 months, 2 years would be better because you have a chance to catch all the hair growth cycles twice and catch any that were missed the first 12 months.

Genital hair removal for SRS should be started at least a year prior to surgery, and probably more. It needs to remain clear for the entire hair growth cycle to be sure its all gone. Any remaining hair may be impossible to remove post-op.

from the trans surgeries wiki on Reddit

Due to hair growing in cycles it takes a very long time to remove properly. If you removed all of it in each cycle (so about once every month or two) you’d need a year. In practice doing a full clearance right at the start probably won’t happen. To be assured its gone you need to count from when full clearance occurs and make sure it stays that way for a year. Worse there’s not a 100% kill rate of hair, so just because its gone doesn’t mean it won’t come back in a year - you’d want to wait another year to catch those hairs on the next cycle. Anything less increases the risk of hair, and how much risk you’re prepared to accept.

from the hair removal section of the trans surgeries wiki on Reddit

The surgeon usually removes or cauterizes the follicles during the surgery, and some surgeons even suggest not doing hair removal at all, but 7% (roughly 1 in 14) of women post-op report having hair in their vagina, and while rare, infections can occur and that can lead to situations you really don’t want to find yourself in.

Anyway, I’m low-key terrified of having hair in my vagina, but I’m also worried my opportunity to get a vaginoplasty is under threat, so I’m hoping to get it done before it’s no longer an option.