I've just been talking to my missus about this, as she's a member of Topcashback.
The length of time it takes to go into your account varies widely - it all depends on the firm you're getting the cashback from and the transaction itself. Insurance policies usually take longer, as the provider needs to make sure that the policy isn't cancelled in the cooling off period and full payment is received.
Also, some transactions don't "track", so you need to be wary of this. My missus isn't too sure why this is the case, but it'll probably say in the help section - she thinks it's down to your cookies on your laptop etc.
A quick check - my missus has had approx £1100 in cashback and a further £300 that didn't track or was declined (for whatever reason).
If you become a plus member, there are added benefits, such as increased cashback, faster cashback payments and priority customer care - it costs £5 per year, but this is only taken after you've earned £5 in cashback.