Well..
I lost mine in Sturgis 3 years ago. We were a long ways from our home base (around 60 miles). I didn't know how long the battery would last, so I pulled the "lites" fuse. I also shut off the oil cooler fan.
Didn't know it until I got on the seat to ride.... that fuse kills: lites, brake lites, turn signals, speedo, tach.
But I made it.... and it was all interstate. And the battery still sounded good once I got to home base. I realize this isn't an exact.... but it kind of gets you in a ball park guesstimate.