I made a thief with 18 in strenght, intending to dual him to a fighter later. Now, I am at the Severed Hand, and he is level 9, soon 10. My question is, should I dual him now, or let him stay thief? Won' it take very long until he regains his thief-skills, and I don't want to be without a Trap Finder too long

I need to know his thief skills AND since Fighters take more time to level up I wouldn't suggest it.

He has 110% find traps, 90% stealth, 45% open lock and... well, pick pocket is low. Haven't put anything into that one.
18 dex of course

Assuming you are playing IWD only (no HOW), (and you're playing on normal difficulty with a full party of six) - its going to take about half of the remainder of the game for you to recover your thief skills. That's based on the assumption that you will do every quest, and each party member will then get about 1.2 million xp.

If it was up to me, I would dual him at 10th level but no later.