When can I give up work after returning after maternity leave?

I have gone back to work after taking 9 months maternity leave. My childcare costs are more than I earn and Im really struggling. My eldest daughter also starts school in September and I need somebody to have her before and after school so I want to give up work. We have worked out that without our childcare bill we can survive on my husbands wages alone. I just wanted to know has anybody else returned to work after maternity leave and then handed in a their notice and if so is there a time limit on how long I have to be back before I can finish. I only got paid SMP so I wouldn't have to pay anything back. Any help is really appreciated! Thanks