There is never a limit on how much energy you can put out into the universe, except when it starts to harm your welll-being. Yes, repeated or multiple spells for the same thing will still work. You are simply adding to the possibility of your change happening in the universe.
A timeframe usually cannot be predicted, however, there are divination techniques that some people use to find out how long until the spell takes effect. Things such as tarot, pendulum, crystal gazing, and wax in water, can all be used for this.