I think the best time to sell is when you have gotten to that point where the home is either hitting to low end of the comarables, there are no further repairs you can make to it, or you have reached that point where you no longer have the funds to maintain it. I personally purchase homes, flip and sell in less than 30 days because I want to get over any possible expenses that may come down the line associated with them. I buy in moderate neighborhoods and look at the potential resell value based on the homes in the area but I also look at if it would be better to just rent the home versus selling.
