Depends on the type/strain of Kratom. 5gr is a decent dose to start with if you have decent-quality stuff, and if it's weaker kratom you can top up half an hour or so later.
Comes on pretty quick if taken on empty stomach (15-30 minutes til full FX for me). The best part lasts about 2 hours for me, but there's another few hours of pleasant mild relaxation afterwards.
The high can be like a (LOW) dose of Oxycodone, especially if you have a Thai or other "energetic" kratom strain. But don't expect too much, the effect is quite mild. Don't go around taking huge doses because you find the effect too mild, it'll just make you puke and shiver for hours.