I’m assuming that Seat will be the same as VW.
In my VW, the ‘service due’ message started being displayed each time I started the car 30 days before the service due date. I’m a low mileage driver on VW’s time and distance service regime (service due at the soonest of 12 months or 10,000 miles), so for me, the message was triggered based on time, rather than miles driven since the last service, giving me a month to arrange for the next service to be carried out and not go over the 12 months since the last service.