My wife received her Spouse settlement visa last week and is now in the UK with me now.

How long is it before she can get herself a part-time job?

If she has a spouse visa then she can work straight away.

Remember she will need to get a national insurance number. She can start work before she has her number as most employers will use a temporary number based on her date of birth until she recieves the real one.

Arrange an interview asap for a national insurance card, takes around 4 weeks.

But as 7/7 said she can start straight away.