Are there any laws that govern the minimum amount of time to test a software?

I've been working at a company and we are going through an upgrade phase whereby we are moving from one version of the software to the next.

We have less than a 2 week (8 working days) testing period to test the software upgrade whereby after the testing period, any requests or queries we raise will have significantly less priority, and will take a long time to fix (if they choose to fix it). We have had them reply in the past upgrades "Why did you not raise that during the testing period?". They are unable to change or increase the current duration of testing period, due to their other work priorities.

In this amount of time (less than 2 weeks), we have users on leave, and other users not having time to test it. The testing duration, start and end dates were given to us (we either use the dates they provided, or have to wait the minimum of another year before an upgrade).

Are there any laws that govern the minimum amount of time to test software, or anything that can help protect us in case we find significant bugs outside the testing period? Is it normal to have this amount of time for testing?

Users are quite dependent on the previous version of the software and leaving is not an option as the transition would set us back a considerable amount of time and money :(