I know people reckon C150's are prone to carb ice but blimey!!
probably a 20 sec burst every 5 mins or so.
thats ever so slightly excessive imo and your fuel consumption will be higher as a result of using that much carb heat all the time!
If you have any carb ice you should get a drop in rpm, and when you remove the carb heat you will get a rise in rpm over what you had selected before you applied it.
I would say a drop of 200 rpm is a bit much but its no reason to start looking for a field, all the C150's ive flown have run rough with carb heat applied.
C250