Minireef 120 - how long to heat up to 25c ? 100 watt eheim jager heater.

So after just getting my return pump and heater in the post today i decided it was time to get everything setup nicely cable management is done everythings looking good just testing everything to make sure im all good for sand and aquascaping next week then onto cycling .. so ive had the heater on since 1-2pm this afternoon its now around 6.30 and its only changed very little temperature wise .. starting to get a little worried thinking i havent gone big enough on the heater .. its an eheim jager 100w rated from 100-150L my display is 120L plus id say theres a tops of 20-30L in my sump .. is this heater going to be adequate for my system or .. sigh .. would it be better to sell it as a new heater and get the 150 watt .. i think there rated to 200/250ish litres .. has anyone used this heater/tank combination ?, thanks guys.