About 2 weeks with 'low risk' foods, and up to 4-6 weeks with 'high risk' foods - low risk being low FODMAP foods, and those that you rarely hear of allergies with. 'High risk' were things like nuts, which many people react very poorly to. I'd add them to the foods I already knew were safe to eat. So for example, I've recently taken to adding buckwheat (which is weirdly a fruit) to my stews and granola after workout. I'm slightly concerned that I may react badly, so I won't add anything new to my diet for a while until I'm 100% sure that it's not causing any under-lying inflammation. I'll mentally keep track of variables like my gym performance, mental acuity, ability to absorb new information, tiredness levels etc. I very much play it by ear though. If I'm especially stressed, or miss workouts, or suffering from any other thing to could contribute to me feeling tired or down, I'll lay off adding any new foods because it will skew 'results'.