Even longer for it to actually power up and start "working." Really excited for it, but feel like something akin to this hubble issue happening with the JW would be disastrous.
(In other words, how expensive would be fixing an issue and trying again actually be?)
In general it's uncommon to build a replacement satellite after an issue. Either you make a backup from the start, or you start over after failure. Making a one-off component often isn't all that much simpler just because you've already done it once a decade ago (and some parts of the JWST should be about a decade old by now). And if you commit to doing that all over again, might as well update the entire design to new capabilities and requirements.
1: https://space.stackexchange.com/questions/48902/are-there-an...
Probably a moot point though: by the time we can get people out to do such an operation, I suspect a reduction in launch costs might lead to a much cheaper space telescope you could just drop off instead.
If all goes as planned, the limiting factor is propellant. And the propellant tanks have the ports for on-orbit refueling, in case there is ever a desire to develop a robotic servicing mission for the task.