I bought some BTC on Coinbase last night and had it sent to Poloniex. I know the BTC wallet address was accurate. Its still pending 11 hours later and doesn't show up in the Poloniex deposit/withdrawal history. Any clue on how long BTC transactions are taking?
You need not worry about that. Coinbase is working on backlog so transactions are delayed. I tried it two-three days ago and encountered similar problem. Usually Coinbase generate transaction immediately and one can track on explorer but currently it is taking time to initate. My transaction took 13 hours to complete.
How long it takes depends on the fee you paid but generally with a sufficient fee it should show up on poloniex within an hour.
Coinbase doesn't allow to choose random fees. It has one fee for all transactions irrespective of size and user.