so.
got a rear tyre punctured today, it was mostly air hissing and barely any sealant. after checking the last top up date, it was the 5th of september, just over four months. no worries, i carry some sealant in a saddle bag for such cases. topped it up (30ml), pumped it up, but it was still not sealing as it should. stopped again to pump the tyre up and by doing so broke the (aluminium) valve stem. o-oh. luckily, after a mini-disaster last year, where both tyres punctured and unseated, i was carrying an inner tube - for similar scenarios. popped it in, pumped it up, cleaned the sealant mess with a baby wipe and reached home no problem.
the moral: use brass valves rather than aluminium, carry an inner tube just in case (i wasn't carrying any for six(ish) years and got away with it).