From How to Meditate Daily by zenhabits:

Start with just 2 minutes. This is really important. Most people will think they can meditate for 15-30 minutes, and they can. But this is not a test of how strong you are at staying in meditation - we are trying to form a longer-lasting habit. And to do that, we want to start with just a two minutes. You'll find it much easier to start this way, and forming a habit with a small start like this is a method much more likely to succeed. You can expand to 5-7 minutes if you can do it for 7 straight days, then 10 minutes if you can do it for 14 straight days, then 15 minutes if you can stick to it for 21 straight days, and 20 if you can do a full month.