rough run
Just a stab in the dark: They generally do not run 'really' rough with carb heat applied. This sound like the mixture is going very rich when the heat is applied. But again this is a guess.
C150 are known for thier carb ice antics. Don't worry too much about over doing it a bit. I used to teach 'on for 10-15 seconds and monitor RPM'. 20 seconds?, whats that between friends.
If you are in known Carb ice conditions, High humidity, temp/dew point close together in the right range, base of cloud etc. , then every 5 minutes is OK.
Just adjust the time intervals for the meteorlogical conditions of the day.
Thease machimes are prone so if you have not already, take a look at the Carb ice guide in lasors. Also, winter'ish time of year is great for picking it up on the ground while taxi/waiting to take off so check again. Especially the
C150.
Even with the best intentions, I have had a couple of rough runs myself in 150/152's due to ice so air on the side of safety. Better be a bit over cautious than look for that field.
I have never heard of a C150/152 having an engine failure due to 'a fouled plug' due to using the carb heat too much . Or any CFI measuring fuel after someone has landed and saying 'your using carb heat too much'.
I have however read many AAIB reports about C150/152, PA28 forced landings due to suspected carb ice.
I have ranted on a bit but hope that helps