My refridge is a Dometric which is propane 'powered' (i.e., a propane flame heats a alcohol tank which boils and then cools) - sounds funny but makes ice cubes and takes me off the electric grid entirely.
I use a 20 pound tank every 3+ weeks just to cool my booze so to speak, and my boat carries two tanks. Using propane for cooking doesn't seem to affect the time
line too much. Tony Smith and his son took a Gemini trans-atlantic on one tank, 23 days IIRC, reefer and cooking.
Those folks that cruise on Gemini's report about the same, 3 weeks or so a tank - some add an extra tank just because it's better to know you have a back up - beats eating cold Dinty Moore.
If you have the room, add the backup.