If you dont enroll your biometric in the first fifteen days, they send you a reminder letter giving you another new 17 days to entroll it.
Why is that, i ll share my personal experience with you. I applied for Tier 1 - General extension on 18th March, received acknowledgement on 20th march. I did not receive any biometric invitation letter till 15th April so I have sent an email to
AppointmentExceptions@homeoffice.gsi.gov.uk and whined about not receiving the biometric letter. I received an email reply back within 3 hours that a new biometric letter will be sent out.
on 20th of April, I received a biometric enrollment letter dated 17th april in the post saying I have got 15 days to enroll my biometrics.
On 22nd of April, I received another biometric enrollment letter dated 18th april in the post stating I failed to enroll my biometric in the last 15 days so i have got another 17 days to enroll it.
The conclusion is, if you fail to enroll biometric in the first fifteen days, the case officer will think that you may not have receive the letter (like in my case, as the first letter that should have been send in march might have got lost in the post cuz its third class post ) and then they send you a reminder letter giving you another 17 days. So dont worry and then submit your biometrics even if u r gone above 15 days from the letter date.