@PhilipDAth is correct, it's a 30-day grace period. There are 2 ways to enter the grace period, one is when the license expires, and the other is when you deploy more pieces of equipment than you have licenses for. You cannot split one AP license over 2 APs for half the time. Every Meraki device is 1:1 with a license, but the license is never tied to a serial number, the license is for one "instance" of that piece of equipment, like an AP or a specific model of switch. If you have 2 APs, you need 2 AP licenses, and with the licensing model as it stands today, there will be a single co-term date.
The simplest example is in the License guide. If I buy 1 AP with a 1 year license and place it in a new Dashboard org, then use it for 6 months, and then (today) I buy a 2nd AP with a 1 year license and add it into the same Org, what happens? License co-term. I would have 6 months of unused license on the 1st AP, plus 12 months of new license, so a total of 18 license-months divided by 2 APs gives me a co-term date 9 months from today. You basically subtract 3 months from the 2nd AP and add 3 months to the first AP to have a single co-term date. A lot of customers love this simple co-term licensing model, but it doesn't work for every customer in every case, which is why improvements are in the works for customers with more complex requirements.
Licensing guide: https://documentation.meraki.com/zGeneral_Administration/Licensing/Cisco_Meraki_Licensing_Guidelines...
There is also a license calculator:
https://documentation.meraki.com/zGeneral_Administration/Licensing/Using_the_License_Calculator