Hiya, ideally leave it some time to settle in, the longer the better, but if you're desperate to get it done, then usually within 4 days or as soon as the plaster has completely set in a soft pink colour.
I would clean up the edges if needed, then with some P180 sandpaper, give it a light rub over, complete dustdown, clean the walls with a light mix of washing up liquid and water to remove all traces of dust. After that, you're ready for painting! 50% paint / 50% water, thouroughly mixed. 2 good coats of this should get your walls nicely sealed and ready for your colour.