There is no fixed time frame for probation confirmation. It all depends on your company policy and your performance during this period. As Bharghavi has correctly pointed out, a confirmation appraisal is usually conducted to determine your performance level just when your probation is about to complete.
If you meet the required level, you may then be confirmed. If not, your probation may be extended. If this is the case, HR and your direct superior must provide you with the necessary mentoring and coaching to bring you up to par. There is also no fixed extension as it depends on your performance gaps.
If it is extended by another 3 months, you will be appraised again at the end of the 3 months. If your performance is now up to par, you will be officially confirmed with an official letter. If your performance is still not meeting expectations, the company has the right to terminate your employment.
The process may seem simple in the eyes of a non-HR, but there is due diligence that HR needs to undertake to achieve the desired outcome for both the company and the individual. Importantly, for whatever transpired, regular communication among HR, your direct superior, and yourself is necessary.
Regards,
Autumn Jane