Qmee, prolific, and Serpclix questions

Qmee – what estimated completion time do you look at ( and what's your strategy as far as what's worth your time, for what pay?

Prolific – those who set up new study email notifications, do they always work for you? I have it so I don't miss much when I'm away from the computer, but I get an email only 30-40% of the time.

Serpclix – is this one where you get more offers the more you search? I've done 19 searches in 6 months, though I don't check it as often as prolific or qmee.

Any other recs are welcomed (besides Swagbucks or mturk).