I agree with Jeremy about talking to your neighbours to establish the best time although I would suggest avoiding committing yourself to a particular time and that time only. I usually find the best times to practise are from say 12noon to about half 6. Really avoid anything after 8pm. The cello and violin are not really noisy instruments (compared to brass and drums) so I wouldn't really get too worked up about being a nuisance to neighbours - in comparison to loud music, telly, arguments, dragging dead bodies across the floor etc (tu)