Title says it all. 600 days is quite a long period of time.
It cannot be altered, no.
It would be nice in the future if the army of the dead are always there rather than being an invading faction but attack at the same time unless attacked first as the army of the dead are obviously always there in the show/books they just haven't marched south...yet! ;)
Honestly the gameplay can get boring if you feel prepared enough to fight them and have to wait :/
They could even have some strange ice fortress in the lands of always winter :p