Guys, let's not confuse the OP with base/S/GTS distinctions here. He asked about a GTS order. Let's stick to GTS orders:
is correct: The average time between actual order at a dealership and physical delivery into your hands is 4-5 months. For example: This time of year, if you sat down with an SA with a build (represented by a code) at a dealership and placed an order in an allocation slot for April -- which should still be available in places -- you'll likely have your car by August, and possibly July with a GTS since Porsche N.A. is prioritizing those orders somewhat.
The key is having an allocation slot that's all yours, with your name on it, at a dealer. You need to do that first. That is what fundamentally determines build date, build schedule, and delivery schedule.
If you move beyond May, however, you're likely looking at delivery in the fall because the Porsche plant where most 718s are built, Zuffenhausen, shuts down for more than a month in mid-July. The secondary plant at Osnabruck shuts down for a time as well during the same time.
Like I said: Get an allocation. That's the key.