I find it's time to charge by the time I get tired of sitting on that hard seat. But yeah, the Zero DS and Zero SR have the hardest seats of any motorcycle I have ever owned.
I used to think my stock 1971 BMW seat was bad!
Since I get long breaks with my Zeros (even at an 8 KW charge rate for my SR) I don't find the seats to be much of an issue. I have ridden my SR from here to Reno many times (99 miles). I charge at half way and that is all the rest I need for the next 50 miles.
BTW, I think the bike can make the entire trip on a charge, but I have always been too chicken to try. There is no place to charge after Truckee until I get home. But that is mostly down hill. My SR does have the power tank. I have gone from here to Truckee many times before getting a charge, or from Reno to Truckee and that being the only charge. But I now normally charge at Cisco Grove which is a good half way point between here and Reno.
-Don- Auburn, CA