I'm converting my NC30 into a daylight MOT / track day bike.
Does anyone have a feel for roughly how long a stock(ish) AGM battery would last in a total loss setup?
At the moment I'm only toying with the idea, but if it can't even do a few 20min track sessions there's no point is there?!

You might be better off posting this in the Rideouts, Trackdays & Events section. That's where the racing fraternity sometimes browse.

You could remove the ignition fuse and place your multimeter across the fuse terminals, set to 10Amp range, and measure the current when the engine is running at various rpm. Divide the battery Ah rating by the current to give the hours, although I would halve the duration again to be sure as the age and state of charge of the battery will also effect the true capacity of the battery.

I would assume that you are going to be runnign tyre warmers? and if so, run an optimate and charge it between sessions,

also wise to have an (anderson?) connector for use of a jump batter when starting it up - that is what takes the edge off the battery.

Any bike without charging would make a shite daytime MOT bike...so make a decision - total loss feels lovely but it is a ballache. I;d not bother for casual TD or for wanting to ever ride on the road again

I'm running total loss on a track bike and I would get about 3 to 4 15min sessions out of the standard gel cell battery before wanting to recharge. Bike will start to missfire at high rpm is a warning sign battery cant supply the current/voltage needed and needs recharging. I've now swapped out the gel cell for a LiFe based battery, runs all day, 7 sessions plus easy. I also run a cheap ebay digital voltmeter on the bike to keep an eye on the voltage level. All that being said, I have only keep the total loss system because I bought the bike in that configuration. I would not recommend running total loss, its very little gain IMO, just a more problems potentially.

Yeah, minimal gains. You’d have better weight-loss by removing battery and just do push-starts