Donchian channels are also often used for stop placement (i.e., set stop for a long below the lowest low of the N last bars)
Regardless of method, there's no one size fits all rule that works for all instruments in all timeframes - so you'll want to spend a reasonable amount of time analyzing your instruments/timeframes of choice to get a good feel of what condition(s) most often signal an end of a trend period (e.g., how many bars to use in the channel, which EMA period works most reliably, etc.).
Good luck.
Regardless of method, there's no one size fits all rule that works for all instruments in all timeframes - so you'll want to spend a reasonable amount of time analyzing your instruments/timeframes of choice to get a good feel of what condition(s) most often signal an end of a trend period (e.g., how many bars to use in the channel, which EMA period works most reliably, etc.).
Good luck.