Of course, the more crank-hours you have the longer it will last. And, it will keep it's "strength" longer. And, of course, this all depends on how long you do your cranking. I often watch people turning and turning their engines over with the electric starter without the thing starting. If they don't start right away, there is usually a reason for that. If this is the case, stop, take a step back and try to see what's wrong. Often you will find that you have a dis-connect on your glow plug, or didn't prime it enough (when cold) or something simple like that.

But, your 7 amp hour batterh should be fine for a day if not abused.

CGr.