Wait until the new Nvidia and AMD GPUs are out to see how that affects the market. I tend to buy every 3-4 years or so but typically not because I need to.
I only replaced my 1080 (That I bought when it came out) with a 1080ti (06/2018) because I got back from a 8 month long deployment at a bare base overseas and my friend wanted a new video card as well. I'm still using it and it does what I need with everything maxed at 2560x1440 on my 144hz gsync monitor.
Saying that I've been eyeing an ultrawide monitor and I may upgrade my 6700k to a ryzen end of this year and I don't lower my graphics settings and I don't game at 60hz so I'll likely get the newest Nvidia GPU as long as the price isn't too crazy.
Supposedly the Nvidia GPUs coming out would be 80% faster than a regular 1080 and DLSS 2.0 may be worth it if you game at high resolutions.
Main desktop video card purchase history w/ full details highest res on latest games
6/5/2018 Geforce GTX 1080 Ti 2560x1440
8/2/2016 Geforce GTX 1080 2560x1440
6/16/2015 Geforce GTX 980 Ti 1920x1200
10/29/2013 Geforce GTX 780 1920x1200
7/24/2008 Geforce GTX 280 1920x1200
5/2/2007 Geforce 8800 GTS 1920x1200
Would have to dig to see what I bought before this