This was driving me bonkers as the .astype()
solution above didn't work for me. But I found another way. Haven't timed it or anything, but might work for others out there:
t1 = pd.to_datetime('1/1/2015 01:00')
t2 = pd.to_datetime('1/1/2015 03:30')
print pd.Timedelta(t2 - t1).seconds / 3600.0
...if you want hours. Or:
print pd.Timedelta(t2 - t1).seconds / 60.0
...if you want minutes.