I don't think that there will be a time where fermentation is still ongoing, but that there will be no off flavors generated. As I understand it, what's going on when a brewer raises the temperature for a time during fermentation, they are allowing for the generation of some off flavors. It will either fit the style of the beer, or if enough of the fermentation is already complete, then any undesirable flavors will be muted.
Depending on the style, the yeast, and your recipe there probably is some point where you could raise the temperature without it adversely affecting your beer. I'm not sure exactly where that would be, but you could monitor the progress by examining the gravity and allowing the temperature to rise after some benchmark is reached between your your OG and FG.
That said, unless you're making some Belgian or farmhouse style I would probably avoid changes in temperature as much as possible, as the change in flavor would be difficult to predict. It's occasionally necessary to kickstart a stuck fermentation, but I would be reluctant to try it if I didn't think I needed to.