Before I obtained a second bottle I would wait until I'd be worried about running out in the middle of a doing something.
By now you should have a bit of a feel for how much you're using. Are you getting nervous? Change it out or buy another bottle. What do they need for a bottle these days? The last I knew they wanted around 50 bucks to own a small bottle plus whatever gas you needed.
I run mine down until I start to see porosity (bubbles) in the weld with my 75/25.
I have a CO2 back up bottle (soft drink tank) that I will use to keep going.
For me it's 2 hours round trip for a fill so I have to plan the trip.
How big are your cylinders? Running it till empty means you'll probably have to grind out a bad weld and if you don't completely grind all the weld out, you'll get porosity and have to grind it out again. Depending on the weld could be a real PIA. Run it down to around 40 PSI or so and you shouldn't have to worry about a bad weld. 40 PSI is a lot more gas in a big cylinder than a small cylinder. Adjusting the regulator may or may not do anything but I've seen a few people turn their oxygen or acetylene up trying to get more gas from an empty cylinder and then forget to turn it back down when they get a new cylinder. This can be dangerous and/or waste a lot of gas. It's always best to loosen off the regulators every time but on a flowmeter/flowgauge, it's very rare to back off the adjuster.
50 psi will get you a lot further than you think. You do not need to worry about you the psi in the tank until you see that the tank will no longer produce the pressure to desired CFM you set. Once you have used a few tanks you will know, roughly, how big a job you can do with what psi you have in the tank. I have two tanks. I ran out once, on a saturday, (the gauge was not working properly) and I will never do it again.