Fruit Flavors are normally good to go or maybe after 2 days they taste even better. Its all subjective as many things in vaping are.. Tobacco liquids taste best anywhere from 1 week to 1 month.
When you are changing flavors that are completely different and you don't think they would mix well, definitely wash out your clearo then maybe do a dry burn to make sure the wicks are dry as well before loading it back up.. but you don't have to dry burn if you don't want. The vape will be a little weak the first few hits if you don't but the water will be vaporized and then your juice flavor will come in to play more..
btw what clearos are you using/talking about?