this is my second playthrough in BG1 and I don't know when I must dual class my fighter
I'm thinking on dual him in level 5 but I don't know if this is going to be a good idea

Waiting until level 6 would give you another proficiency point, and waiting until 7 would give you an extra 1/2 attack with any weapon you've got points in, though the latter might not let you get your fighter abilities back until BG2. I'd have to check the XP tables to be sure of that, though. What were you thinking of dualling into?

Edit: Assuming you dual at 7, any dualclass but Fighter/Cleric would let you get your fighter abilities back before the BG level cap, though you'd have to get as much XP out of the game as you could. Any dual class would work if you dualclassed at 6.

I'm thinking about cleric

Dualling at 6 would probably be best if you're not planning on going into BG2 (or if you really want your fighter abilities for the BG endgame); just make sure you put all of your points into weapons the fighter can use, as any points into swords/bows/etc. will be lost.

I not planning to go to BG2 with my fighter because I'm halfway through BG2 with a paladin.

Level 6 is the highest fighter level which still allows to reach cleric level 8. But if you don't want to wait until chapter 5 with casting priest spells dualing at level 3 already offers most benefits.

True, but level 8 cleric doesn't really offer any benefits over level 7 except being able to access your fighter skills earlier, at least in my opinion.

Cleric level 8 is better than level 7, you get more spells at higher casting level. But it's irrelevant for the dualing question since you can reach level 8 if you can finish dualing, if you start dualing at fighter 7 you can't finish it within the XP cap.