How much exercise is too much exercise?

Recently I have started to fight a bit with dumbbells (2 and 3 kilos pairs).
As well as doing sit ups.
I want to get rid of a small belly (think 5 months pregnant), and I want to tone the muscles in my arms, and my shoulders, some.
So far I have been finding out on my own which (dumbbell) exercises feel good (that is, if they ‘pain’ me I think they are working).
The sit ups I do as much as I can, before I just can’t get up anymore.
Now my question (if the goal is to reach a flat belly and toned arms and shoulders quite quick): do I do my exercises once per day, or as many times per day (start again after all the muscles feel painless again)?
Extra question: do these elastic expander ropes do anything?