If it’s a one off filament that I don’t have more of and don’t plan to restock, I just use it for bed leveling tests and the like, or on prototype prints that can have multiple filaments. For ones that I have more of, I just keep an eye on the printer when it’s running low, and shove the new filament in as the old one runs out. I don’t even bother with a runout sensor or pausing the print, I just let the old one fuse with the new inside the extruder and call it a day