I'll try to answer beginning with your last question. How long can you use your inverter without recharging the batteries??? Well of course, until the batteries are dead.
But seriously, if you're running a fridge, BE PREPARED!!! What you need to do is know the operating current
....I presume on 12vdc and the duty cycle. For instance, if your fridge draws about 6 amperes @ 12 vdc, you next need to know the duty cycle which is the ratio of on time to off time. 50% on, 50% off is a duty cycle of .5 so the total drain on the battery
is 6 amperes times 0.5 or 3 amperes. Now this can get a little complicated. Battery
discharge hours depends not only on charge but at the rate of discharge. A 100 ampere hour battery may provide 1 ampere for 100 hours, at 20 amperes it may only provide 3 or 4 hours. You need to check your battery's performance curve.
I use a 1 kilowatt Xantrex pure sine wave inverter for my 8.3cu ft Summit fridge that has a self defrost mode and an interior
light. The reason for the high wattage capability is to be able to handle the inrush current
when the fridge starts along with the self defrost. Be careful using an inverter. Many motors do not like pseudo sine waves. Personally I would try a cheapo from Harbor Freight something for less than $100 in the 1 kilowatt range. If it works your fine, if it does not, return it.
EDIT: WATERWAYGUY's post above mentioned the inverters in his URL pointer may not be marine
rated. I MUST point out that if your place an inverter or for that matter anything in a bilge
, it must be marine rated. My inverters are NOT in my bilge
. Give that some though.