Hi,
Soon after I bought my 89 cab with 91K on the clock my local garage said that the Pierburg carb would last around 150K before playing up. Sure enough at 160K it started to give me gip and was replaced with the Weber conversion. Although this isn't cast in stone it may help as a guide to how long yours may last.
Problems will be obvious, generally running like a sack of poo, problems starting, etc.
You may also get problems with the cold/hot air box that sits halfway between the carb and the air intake that snakes down the left of the engine bay (facing the car) and ends just behind the rad. Over time this either sticks open so you'll be drawing cold air in at all times of the year, leading to carb icing in the winter. This is easily sorted by screwing it permanently shut so you draw warm air off the manifold. Doesn't affect the day to day running so as you'd notice.
You can replace these, but from memory I think they are around £200-£300, so it depends on how flush you are feeling. Other than that, carbs are fine!
As a testiment to VW's quality, in the past I maybe didn't look after my engine as often or as throughly as I should, and left problems get worse rather than sort them sooner. A few years later and with everything sorted, the garage ran a compression test and it was 170psi on all four cylinders, which on a car with nearly quarter of million miles is pretty special, according to them anyway! While I'm not suggesting you can neglect the engine, it seems to be quite a sturdy unit so hopefully you should get years out of it yet.
Good luck!