How long is a radiator vent supposed to last before replacement?

If you have not, check out this thread.

Why replace? Are they stuck open or closed?

That's a little like asking how big is a box. Too many variables. However, they should last for years — if they are never abused. The most common abuse is too much pressure. Yes, I know, some are rated for 15 psi — but that may be a one shot deal. The other abuse is frequent cycling. In this they are like switches, which are rated in cycles, not years.

If an air vent has been tested to last 10,000 cycles and your boiler operates with about 2,000 cycles per year, then about 5 years.

If your boiler is oversized and heats the home too quickly then the house cools quickly as a result of old construction, then you system may cycle 4000 times a year, then the vent may not last as many years. The same amount of cycles but less years.

How long do your tires last in years, not miles… My tires last over 5 years now that I'm retired and don't drive as much as I used to. The last set of tires were replace due to dry rot on the sidewalls. still had plenty of tread left. Go Figure!

I think the ones like Dole and Hoffman with a capsule inside can be damaged by excessive pressure, deforming the capsule. The Bigmouth too if it is Steam pressure, I think the Bigmouth may survive depending on the water temperature, since if the water temperature is cold enough the water would just run through it, not much restrictions.

If a vent is operated in a distribution system that has high pressure example: 4 to 6 psi, there are things going on in that system such as dirt and debris entering the vent clogging the orifice closing pin [canister] or head [bimetal strip] resulting in it venting steam.

The most common failure cause I've found was dirt and mung. High pressure and poor piping carry over debris laden water from the inside which lodges in the vents. Paint and dust clog orifices from the outside. Vents serving clean piping filled with dry steam seem to last forever.

Some brands fail by other means also. Heat timers have an internal mounting that fails. Hoffman's sometimes lose their alcohol charge.

But clogged vents can often be boiled to clean. I have many that have seen well over a century of service and still work fine.

Yes it would depend on how open the vent was and of course the size of the vent, but there is zero doubt that a vent passing steam will have a pressure reducing effect, just maybe not noticeable especially in an oversized system that you surely have (as almost all of us have)

Yes maybe