how much time do you have to invest to make it worth it do you reckon?
That depends on you and what your time is worth tbh.. if you're a student doing something easy/plenty of free time then you can start with very little and it might be worth spending time grinding away at various offers - easy money compared with a bar shift at the students union or shelf stacking in the local Tesco etc..
If you're in full time employment and earning a decent wage then you might only bother with the low hanging fruit in the form of some of the generous sign up offers and a few of the ongoing ones.