I have the original instructions for a Synchronar 2100 Mark 111 which gives the recharge times as....
Bright sunlight: 10 hours.
Bright overcast sunlight: 20 hours.
Dark overcast sunlight: 50 hours.
The above through a window: Double to triple the time.
100 watt bulb at 5 inches: 30 hours.
60 watt bulb at 3.5 inches: 30 hours.
15 watt with reflector at 5 inches: 30 hours.
Fluorescent lamp: Useless for charging.
Hope this helps.