About 5 months, sometimes more, sometimes less, a top up costs less than a tube anyway, and I’d get through a lot of tubes in 5 months! Strip out as much of the gunk as possible.
CO2 seems to cause it to ‘curdle’ relly quickly (guess as it’s acidic), no need to wory about sourcing nitrogen, the air is 80% N2 and oxygen’s neutral too. There’s reasons race cars use N2 but they’re mainly things like saftey in fires (a tyre full of nitrogen in a fire might put itself out when it pops rather than feed air to the fire) and controling moisture content (as by definition bottled N2 is dry). Seeing as bikes arent subject to either problem then it’s an expensive waste!
(**** ME it was hard work getting NobbyNic 2.1 UST tyres on the rims!!!!!!!).
Get the bead into the centre of the rim and start opposite the valve (which would prevent the bead being in the center if you started there). Using tyre levers etc makes damaging the tyre-bead or stretching it more likley and therefore less likely to form a reliable seal with the rim.