If you will give me the exact model of head unit you are thinking about, i can give you an estimated amp hour usage. Include model, and if you have another depth finder add that model. If you are going to be running on your starting battery you need to think about other current draw elements (livewell recirc, nav lights, bilge pump, accessory items).
But to really do it right you need to do a amp hour calculation on your boat. Here are some ballpark figures. Basically it is (battery size in AH(amp hour) divided by (current draw of your electronics combined).
93SV and Livescope - 19AH for 12+ hours of operation. 30Ah for 18+ hours. 32Ah for 20+ hours.
10" Screen and Livescope - 30Ah for 10 hours. 32Ah for 13+ hours. 48AH for 19+ hours.
12" Screen and Livescope - 30Ah for 8+ hours. 32Ah for 10+ hours. 48Ah for 15+ hours.
Search online for a battery run time calculator. Note: I added a 55ah duracel agm (sealed) battery in my boat to run electronics. Two things, I wanted a noise free dc voltage to my electronics and I didn't want to deplete my starting battery. My boat setup would have depleted the starting battery enough that it would prob not start the motor after 12 hours (from depth finder and the mega 360 alone). I run power pole connectors on all 4 of my batteries, so I can move electronics to another battery if needed. The charging system on some motors have tons of (RFI: radio frequency interference). Basically a sinewave signal on a dc voltage power circuit. I am a ham operator and RFI is the Achilles Heal to all radio systems . Many people are using the lithium batteries for tools to power their units, which are quick to swap. Might look into this.
Here is a web link that has some specs on amp draw per unit.
Linky