Migration of Sharepoint 2010 to office online(365)

Hello Everyone,

I am doing estimation of one project for migration of Sharepoint 2010 to office online.

I am new to sharepoint and most of my experience with .net.

I am going to use sharegate tool for migration.

i have two content databases( 500gb and 150 gb) and 5 site collection, few webparts(16), custom pages.

can anyone guide me how much time it will take for migration of this site

also which points i need to consider while creating estimation for this project.

what challanges i need to consider and start working on it?

Thanks,

Chetan R