Hey. I am replaying BG from 1 till tob (playing BG trilogy) as mage this time. Human, chaotic good mage.

i will keep Imoen in my party through whole game (if she is available that is). Therefore should I make her a mage, if yes, when? or should I keep her as thief?

Well I definitely recommend dual-classing her regardless of your party setup. But with BGT, your companions carry over from BG1 to BG2 directly, so you'll have to think pretty hard about what level to dual-class her. In vanilla BG2 she's a thief7/mageX, which I think is the best way to use her since she can max out the two most important skills, pick locks and find traps, and can use illusion spells for sneaky stuff. However, that can make BG1 very hard because she'll be a puny mage for a lot of the endgame. You could also wait until level 8, which will get her another proficiency point and more skills but will make stuff even harder, and might not even allow her to reactivate her thief skills by the end of BG1. (I'm not really sure how BGT handles the XP caps for the different games.) It's possible to get locks and traps to 100 by level 6, but only just barely, so you could do that too.

I'm playing more or less vanilla BG1 and I dualled Imoen to a mage at Level 5. However, I'm entering the Cloakwood and she's still a Level 4 mage. I like having the extra magic oomph (i'm using the 'canon' party of Khalid, Jaheira, Minsc, Dynaheir, and Imoen), but I'm starting to miss the thieving skills. I may try and grind a level to bring Immy more up to speed.

Keep her as thief until she's level 11 (and can set some traps, if you don't like setting traps level 10 is enough) or even forever, there's no need for a second mage in the party but playing without a thief is painful. Dualing to mage works much faster in BG2.

she is level six at the moment. i need trap spotter (i don't set traps myself really cant be bothered with the micromanagement)
in cloakwood at the moment just beat the spider queen.

so not sure whether ill dual her because i need a thief for the tower.