Depends how high. Ammonia and nitrite are inevitable during a qt
. The goal is to fight them and keep them down.
I have 2 heni butterflys (3") in a 10 gallon qt
tank right now. I change 2 gallons every day or every other day (20%) depending on my tests. I am usually battling a .25-.5 level of ammonia. If its higher you would change a little more. Don't know if there is a "rule of thumb", but its kept my fish healthy.
You also want to be careful not to change too much as it can stress them. It's a fine line, you'll figure it out.