In most countries, the trademark opposition window will be 2-3 months long. However, there are a few exceptions, such as the USA with only 1 month on one end or Vietnam with 11 months on the other.
Bear in mind this is the standard length of the oppositions window. If an opposition is raised during this time and the proceedings start, they can take from a couple to several months. For example, in China, oppositions are open for 3 months, but if an opposition is raised, the average review time is 11 months, and the timeframe for a ruling on the opposition is 12-18 months on average.
Also, remember that in some jurisdictions, e.g. the USA, the opposing side can ask for an extension on the deadline.
For these reasons, it is crucial for business owners to perform a trademark search to learn about similar trademarks whose owners would have legal grounds to raise an opposition. Even if oppositions cannot be fully avoided, this information helps plan other business operations around the fact that the trademark may be approved later than expected.