Dad’s short answer: Figure on somewhere between forty minutes and an hour and a half depending on mode and machine. But run time is rarely what ends your session — the tanks almost always run out before the battery does.

The numbers, and what they mean

Manufacturers quote their run time in the lowest mode, which is not how anybody uses these. Eco or low mode will get you the advertised figure. Auto with a dirt sensor ramping up on dirty spots will cut it. Max mode on a big floor will cut it a lot.

In practice the machine will run longer than your tanks will hold water. On most units, a full clean tank runs out before the battery is anywhere near flat. So unless you're doing a very large space in stages, run time isn't the limiting spec, and I wouldn't let it drive a purchase decision.

What actually matters on the battery

Whether it charges in the dock. Nearly all of them do now, and a machine that lives on a dock charging between uses effectively always has a full battery when you want it.

Whether the dry cycle draws from the battery. Some machines run the hot air dry off battery power, which means a long dry cycle after a long clean can leave you low if you wanted a second run. Worth knowing if you're doing a whole house in two sessions.

And whether the battery is replaceable down the road. Like any cordless machine, the battery is the part most likely to end its life, so it's worth checking parts availability before you buy — same advice I give on robot vacuums.

Dad’s Take: Don't shop on run time. Your tanks will tap out long before the battery does.