At 13.3 it will take forever. You haven't said what model converter. If it's a Progressive Dynamics it may have a boost mode that goes to 14.4 and that would help a lot. You battery is never going to reach the proper specific gravity and will be damaged.
If you seldom dry camp or only for a weekend a good converter may work for you.
If you often dry camp for extended periods, no converter will work well for you.
If you have lots of time to be on shore power a slow convert may be ok
If you want faster, get a real battery charger
A real charger
