How much time do you invest into learning full songs?

I mostly play classical guitar and fingerstyle songs like Careless whisper arrangements. Usually I sit in front of pc looking at tabs and playing along. Two hours can pass really fast like that :D but the problem is that when I try to play without pc, I suddenly realize I dont know shit, quite often i even dont recall what chord should come next.

obviously depends on the songs, but how much do you actually invest into learning songs and not just playing around, and how long does that take you?