When the query writing starts to get in the way of the actual writing, then you should start to rethink things a bit. You are not going to improve your writing by writing more queries. Market-wise, sure, keep track of who buys what and what they are looking for. But markets change, editors change, magazines fold, tastes change. That happens more than you think.
Queries are certainly invaluable.
Just remember to keep writing too. There are two schools of thought on a writing routine. When I first started writing, I forced myself to write for an hour a day. Years later, I shifted to trying for a certain number of pages per day. Now, I don't shoot for either.