Microscopic unless you have a lot of visitors. You will get much more from ads. However, if you give them an incentive to run the miner for longer durations (like giving them in-app rewards) it may be good.
Not just a lot of visitors, bit visitors that stay on your page a long time. Like the UFC video site that was caught out.
50 desktop users an hour a day for 2 years to meet the min payout.