You should be able to find how long your set up by figuring how many amps your finder draws (available in the owners manual or online) and how many amp hours your battery is rated. It sounds like the battery would be the unknown variable in this case. Unless the battery lists a specific number, i would not assume it is an 8Ah simply because it is bigger than a 7Ah. I am pretty sure that the GPSMAP 178 draws 1.2 amps per hour and needs a minimum of 10 volts to run properly, (but don't quote me) so you could expect 6 1/2 hours of use from a properly functioning 8Ah battery. With the led lights, the unkown battery Ah and life remaining, and an amature giving electrical advice, there are a lot of variables that could affect your system. Good luck.
