If your employer is told too soon to stop making deductions from your pay and this leads to an under-deduction either:

• if it's less than £120 the Student Loans Company will ask you to repay it directly to them or
• if you don't want to pay direct or it's more than £120 the Revenue should tell your employer to start making deductions again.

It doesn't matter how long it takes you to repay the loan.