It will vary with the sealant, with the tubular and with the environment. Once the sealant is inside keep that tire inflated forever. A flat tire with sealant inside leads to a tube that is stuck to itself. Then add air and the tube rips. This is a reason to not add sealant until you flat. If you do high mileage you will wear out tire first. If you have an off season it becomes an issue.

I have done this and gotten away with it for long periods. And also destroyed good tires. The risk is always there.

A tire with sealant will hold air longer.

Everything is two piece valves now. If the valve gets clogged remove and try holding it under runing hot water.

Good quality tubulars flat as seldom as good equivalent clinchers. Never worked this well in the years they were both popular and got a bad reputation. If you are in goathead territory or otherwise get lots of flats sealant is not enough to help.