UKBA states that you have 3 months.

How can you change your personal details on your biometric residence permit?

You must tell us as soon as you can if:-

you change your name (for example, because of marriage or by deed poll);
you change your gender, legally or permanently;
you change your nationality; or
your facial appearance changes significantly.

If you change any personal details listed above or any details shown on your biometric residence permit, you must apply for a new permit within 3 months using application form BRP(RC). If you do not do this, you may have to pay a financial penalty of up to £1,000 or we may shorten your permission to stay. You can download the application form and guidance from the right side of this page.
Here's the UKBA webpage