Hobbico "overnight" 12v changer that came with my starter box 7ah batt charges at 600ma and they spec 12-16hrs charge time on batt box as a normal charge.

Car trickle charger is usually 2-5amp and a motorcycle trickle charger is usually 1-2amp depending on size. I def would not try more than a 1amp charge rate, the float charger should be fine unless its for a car - they come in diff sizes. I keep a Battery Tender on my motorcycle batt and its 1.25a. The one on my starter box batt ins the Battery Tender JR and its rated for 750ma. The one they sell for cars (battery Tender Plus ) is 5amps.

Check the specs on your friends and make sure its not too much. Three of these in parallel should do a good job as a box batt.