About a month before you are going to return from your trip, it is a great time to download offline maps to your phone. 
 
They usually expire in a month.
 
I download a number of maps to my phone to cover the area I am travelling to. This way if I need any directions, I can leave my phone in airplane mode and not use data. You can use it to mark where you need to return to and navigate the destination you are visiting without worrying about getting lost while saving those roaming charges.
 
Here are two links to articles explaining the process: