Boosting time doesn't make a difference and you use them just like expert/veteran packs.
Also, it's highly advised to NOT boost gunners, BOs, and anything else that caps easily.
They are best used on things such as reps, engs, and pilots which are very annoying to cap.

It doesn't matter when you boost the sailor

once a sailor is boosted it cant be reverted back. whatever stats he has at any lvl are
boosted. so boosting at lvl 1 compared to 120 provides no benefit for either lvl.

to answer your question, boost when you want to. if you want the benfit of the boost now
then boost him now, if not then wait until later. the only benefits you get are that the
sailor is either boosted or not, there is no gain in stats or ANYTHING for boosting
earlier or later.

Once you boost a sailor, that sailor is boosted permanently. You can do this at any level.

If it's a question of spending money on the sailor, though, you want to vet him before you
boost. Vets provide the skill, and the more skill the sailor has, the larger the boost
will be.

It does NOT matter when you boost him. Its a 20% increase to whatever stats he WOULD have
if he was not boosted.

Example:

We have 2 gunners. Both have the same stats and growth. We leveled both at the same time
and both have 1000 accuracy and reload. If we boost one of them he will have 1200 acc
andreaload while the other will still have 1000. now we level them for a few days and they
leveled up (more ability) the sailor that previously had 1000 ability now has 2000 and
theboosted one got 2400. Now if we boost the unboosted one they wil BOTH have 2400 ability.

In other words. Boost as soon as you can. It makes no difference to sailor skill but will
help you in leveling.