They have updated the timetable here

So here's what I can gather

Service as it is now every 20 minute

From 6pm the service will become every 15 minutes

Every 30 minute between Gateshead & Whitley Bay & every 30 minutes between Newcastle & Tynemouth making it every 15 minute for a 1

That will run till 9pm to which the service will run to every 20 minutes again with short tips to Tynemouth then at 11pm it'll be 30 minutes with trip one will finish at Tynemouth & trip two last of service will finish at Percy Main. (I'm reading this from Gateshead to Whitley Bay)