Forex Signals

There are commercial robots, as well as free ones, and you could even design your own without writing a single line of mql4 code…but how often, or how do I know for sure that I need to optimize/re-optimize a forex robot using historical data in metatrader, since the market keeps changing? and what would be the length of time to use to optimize for 1M, 5M, 15M, 30M, 1H, 4H, Daily charts? thanks!