how long is the B1 English test valid for?

Some recognized test qualifications only last for 2 years. You can still use a B1 level qualification that you took more than 2 years ago in 2 situations.

1) Applying for citizenship
You can use a B1 level qualification that’s run out if you’re applying for citizenship and it was accepted when you settled in the UK.
2) Applying to settle in the UK
You can use a B1 level qualification that’s run out if both of the following are true:

It’s on the current list of recognized tests.
It was accepted for another UK immigration application, for example when you got permission to enter.