I have recently (half an hour ago) managed to change my SSD from IDE to AHCI mode. This should also enable TRIM, which in itself can also improve performance, right?
How long should it now take before I start seeing the effect from TRIM?
TRIM commands will be issued immediately. However, space that is logically empty on the drive that hasn't been TRIMmed won't ever get trimmed until that space is used and then made free again.
If you want to force it, just find the largest file you can and make copies of it until your drive is full. Then delete all those copies. Since TRIM is now enabled, the free space will be TRIMmed when you delete the copies. There are tools to do that automatically too.