Данные
df=pd.DataFrame({'ideal_appt_time':['2020-01-06 09:00:00','2020-01-06 09:30:00','2020-01-08 14:00:00','2020-01-12 01:30:00'],'service_time':[22,15,42,35]})
Другой выход
df['finish'] = pd.to_datetime(df['ideal_appt_time']).add( df['service_time'].astype('timedelta64[m]'))
df
ideal_appt_time service_time finish
0 2020-01-06 09:00:00 22 2020-01-06 09:22:00
1 2020-01-06 09:30:00 15 2020-01-06 09:45:00
2 2020-01-08 14:00:00 42 2020-01-08 14:42:00
3 2020-01-12 01:30:00 35 2020-01-12 02:05:00