Hi Martim,
If you are familiar with setting up SAPGUI Installation Servers, not much time. However, if you're doing it for the first time, you will need to plan on some time for troubleshooting your scripts and getting them just right, and also for pushing out the install. With basically just the manuals to guide me, and also with a wide variety of starting scenarios to plan for, I spent quite a lot of time on this -- easily a couple months (with other production support tasks as well). However, I think I can save you quite a bit of that, as I detailed exactly what I did in a series of documents, the first of which you can find at SAPGUI Installation Server Part 1 - Getting Started.
Hopefully you'll find that helpful.
Best regards,
Matt