I find that roadadvisor is a lot more optimistic than I am! While I agree with mi amigo about customs not taking very much time, if you are arriving the same time as several other flights, it could be crowded in passport control (migracion) or it could take some time to retrieve your baggage. As we all know by now, air travel these days can be a difficult thing and if your flight takes off late, for instance, you would need to factor this in.
Part of the answer has to do with which airline you are flying on - if it is Sansa, it's in a part of SJO about 5 minutes away from the main terminal. If it is NatureAir, they fly out of the Bolanos airport in Pavas, a suburb of San Jose, and the trip there can be anywhere from 20 minutes to an hour, depending on traffic, etc. If it were me, I would allow about 2 hours between flights if leaving on Sansa and 2.5 or more if leaving on NatureAir.
You might whiz through everything and sit twiddling your thumbs and cursing yourself for taking my advice - and cursing me - but I would hate for you to experience some delays and just wave bye bye as your plane takes off for Tambor.