It doesn't matter if you keep the surfer on for the requiered 100 credits or the whole day, you'll still earn the same ammount.
When you start and you don't invest you have 10$ in the account, that means every day of the month you surf those needed 100 credits you get 1% = 10 cents, when you either invest more to the program or wait until your earning accumulate to the next level and you roll over the money, then you start to make more per day and per month.
( i can't remember all the new options so I'll go with the old formula)
Roll over 10$ of your earnings (or invest) and then you have 20$ in the account (10$ from the original sign up), with 20$ you make 20 cents per day when surfed 100 credits, again next slot in then you make 30 cents per day which already equals to quite nice 9$ish per month etc etc
I'm going towards 40 cents per day, I now have 8.5$ in my account. Never invested anything more than my time, I let the money grow and rolled the cash into my upgradition.
If you have the money to invest or the time to wait that it builds up yes it's a great program and I'm very happy with it!