As long as is needed to transition away from burning fossil fuels. You can always put more in high altitude. If half the comments here are right and climate change is truly an apocalyptic threat, then acid rain and global dimming would be worthwhile tradeoffs.

If climate change isn't extremely bad, then maybe it's not worth it. But at some point, if it becomes existential, then all options are on the table.

> How long would that temporary measure help?

As long as you do it.

> What would that do to acidic rain, global dimming, ecology and agriculture.

Well, it's a trade-off. Is sulfur worse than global warming?

> Masking the problem will only make it come back harder

Geo-engineering is not an opportunity to stop phasing out fossils, but a way to get more time for better technology to spread.

> Masking the problem will only make it come back harder when measures don't scale up anymore.

If (e.g.) China stops the US can start, or if a small country can't scale up enough more countries can join in. The whole "termination shock" thing is highly overrated, it's solvable with next to no communication at all. Decision rule: if not enough is being sprayed according to your models, make up the difference yourself.