Whether or not it's "time to upgrade" depends largely on what you're doing.
Do you do a lot of video editing or do you edit a lot of RAW hi-res photos? If so, moving to 32 gb ram would be noticible. If not, 16 gb is fine.
Do you game at 4k? If you game at 4k, adding another GTX 1080 and running SLI can give a HUGE boost to 4k gaming, especially when the game supports it and scales well with SLI. Not all do. If games you play scale well with SLI and you play at 4k, then adding a 2nd 1080 would be worthwhile. Otherwise, no.
The CPU is fine unless you're editing video for money and time is an issue, you would want to move to a more modern higher core count CPU. If not, then what you have is fine.
Regardless of WHAT you're doing, upgrading your old spinning magnetic platter hard drives with Solid State drives (SSD's) is ALWAYS a worthwhile upgrade. Now that you can get 1 TB name-brand SSD's for $150 (or LESS, WOO HOO), the move is WELL worth the cost. If you're on regular hard drives now, chuck em in the garbage and upgrade. I have 4 systems currently running, and they ALL have SSD's in them of varying sizes, mostly 500gb and 1 TB jobs. I don't run a single mechanical hard drive anymore, even this laptop I'm on now has a 500GB M.2 drive and a 1 TB EVO 860. I PROMISE you, this is the upgrade you want to make immediately if you haven't yet.