Hi Tortoise the battery life is stellar on the Garmin Inreach SE if you are using it as a preset message at say 8am,12 am and 8pm. Turned on to send a message and off at all other times and you would get much more than 8 days. I would hazard about 25 days, perhaps more. When it is turned off there is no real battery drop. Leave it on (say all daylight hours) and you would probably send those 3-4 preset messages per day for 6-8 days. Someone else will be able to comment on that.
My use is as above -presets (both email and text, in the one message) to my daughters and my email on the el cheapo Safety plan. If you check this thread
viewtopic.php?f=21&t=27083 it might offer you some more info. Full charge from dead 2 hours-confirmed.
For this sort of device a "good" size screen and a good chunky size to get a grip on should the faecal matter strike the fan. The mini I feel is just for communicating I would be very concerned trying to use that if something was wrong and I was agitated.
FWIW I would not get the Mini, as I am using mine as an emergency item and I believe the toggling thru' the small screen is a nightmare-there is an excellent review of the Mini on BPL which confirms this. Sending a message on the Inreach (Garmin) can be managed should you have to and are capable-it is the old school texting a la early mobile phones.
I basically use this a a PLB that I can communicate with and so far have only sent preset messages of which you can compose three.But you can text away if you want.
Anyway good searching-downunderpilot/aviation shop or something had them for the best price I could find at the time.