From my experience, there is no point at which you can no longer complete quests. many people have talked about doing the critical path (ie-Imoen)first then completing the quests. A fair warning about doing that-Once you enter that process, there are a number of other things you have to do before you can get on with the original quests and it takes a while.
What you really need to decide is how important Imoen is to the party as a whole and how much experience you want before going after her.
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