How much tretinoin should I get?

Might be a dumb question, but how long does a 20g tube last? And for how many months should I use it for? I’m trying to kickstart my gains again, as I plateud for the past few months after 1 year of pretty solid gains. (Went from complete babyface to solid goatee but cheeks are very weak).

Are 2 tubes (2x20g) enough for 5-6 months?

I’m ordering it from Romania as an Austrian, and shipping is crazy expensive (10€).

Can’t decide wether to add anothet tube.

Also I’m concerned that my face gets destroyed in the summer while on tret (apparently you get very sun sensitive)