We do not know how long it will take USCIS to reschedule the appointment. However, as noted above, since the results of the fingerprints are only good for 15 months, if an Adjustment of Status application is pending longer than 15 months (likely for quota backlogged cases), it is likely an applicant will need to be re-printed. Therefore, given current processing times for Adjustment of Status cases, in addition to quota backlogs, a delay in the initial biometrics appointment is unlikely to delay the ultimate adjudication of a case, as the biometrics will still be completed well before the Adjustment of Status application is ready for final adjudication.
Although rare, it does occasionally happen that USCIS does not properly record the request for rescheduling of the appointment, and erroneously determines that the applicant has missed his/her biometrics appointment. A denial notice is then generated for the Adjustment of Status application, stating it has been denied due to abandonment. We must then request that the Adjustment of Status application be re-opened and provide proof that the reschedule request was submitted prior to the biometrics appointment date, or that the individual attended an earlier appointment. USCIS will then re-open the case.
Note that multiple requests for rescheduling an appointment without valid justification could result in denial of the application.