u need around 3 to 5 hours to get the 250 gamerscore.
signal is the best episode yet i love it and be warned u will die it is difficult on normal also it is EXTREMELY difficult on nightmare. i finished nightmare difficulty for signal and i died around 35 times.
That said this will likely last you 1-2 hours though reason to go back and I think worth your time.
Some people it takes them 4 hours and others can rush it in 30-40 minutes, etc. Too open based on how you play but enjoy yourself. It's about the length of a regular Alan Wake chapter but a shorter chapter. IN FACT you know the Mirrors Peak half of Episode 3? It's about as long as that, episode 3 in my opinion always felt like two episodes stuffed into one.