Limit of Generating Script for Schema Only from SQL Server 2012 to SQL Server 2008R2
Schema generation will totally depend on the amount of objects (tables, SPs, triggers, indexes, etc) in database.

Schema is just a skeleton of the database without data.

If you are concerned about using SSMS GUI, you can use PowerShell - which will definitely scale better - and can be automated easily as compared to SSMS GUI.

I have tried scripting out schema of 1TB data warehouse database without any issues (as I said previously, the more objects you have, the bigger script it will be - and slower too (better to use PowerShell)).