Cell standby use about 35-50% of total battery?
What matters is, what is your total battery life, if your phone lasts 2-3 days, then 30% is reasonable, however if it only lasts 8-10 hours, then it isn't.

If it is the latter I would recommend clearing your cache (and davlik cache if you can) and or performing a factory reset and seeing if the problem persists, also try using a different sim card on another network.