# of nights before 2nd Seam seal

There are really too many variables.
Direct exposure to sunlight? Shade? Temperature? Lots of rain? and so on.

Just do it when it starts looking like it is leaking.

I've never seam sealed a tent or tarp twice. If it starts to leak, then certainly, but seam sealing (if done well the first time) is not likely to fail catastrophically and leave you cold and wet. I believe most manufacturers are saying that it only needs to be done once.

"I've never seam sealed a tent or tarp twice."

+1 to the above statement.

I have, after several years of use, had to seal some pinholes in the fabric, not on the seams, which were perhaps caused by camp fire embers.

I have also had to follow up (the next day) an exterior seam sealing job with an interior dab of sealant on one small seam section when I subjected the original sealing job to a backyard sprinkler test and observed some leakage.

If you need to seam seal your tent again, chances are it was not done correctly the first time.
The idea is to fill the stitch holes not to slap a thick coating (that can peel off) on top of it.

When it leaks, if it leaks. Any failure won't be catastrophic (at least at first) so I wouldn't worry about pre-empting too much.