Operating fish finder using lithium battery

[Kent]

I have always had my fish finders attached to my starting battery.  When I am vertically jigging, trolling or fligging I like to leave the fish finder on.  If I leave the fish finder on longer than ~3 hours I get concerned about running the battery down too low to start the outboard, so I usually switch off the fish finder.  I always carry jumper cables and a jump-start battery, so I am well protected, but I still prefer to not take any chances.  I am going to begin using a separate lithium battery to power my fish finder.    I currently have a Humminbird HELIX 9 Chirp MSI GPS G3N fish finder.  I'm not sure how powerful of a battery that I need to install.  I realize how bright I have the screen has a major impact on power draw, so I try not to have the screen any brighter than necessary.   Does anyone have any experience in how long a 50 Ah would power my Humminbird?