According to CDPR patch 1.6 will be coming "sooner than we think" no idea what exactly that means but considering that statement is already over 1 month old.... my best guess is end of the summer ??? maybe? As for how big of an update that will be? also no solid answer, best guess the transmog system they teased might drop with it, probably some free dlc(Weapons/cars/etc...) hopefully some random encounters or new side hustles. NG+ is a possibility but again that's mostly speculation with a side of hope.
As for a "Major" content update, 1.6 might bring some cool unexpected stuff and some good content, but the next BIG story expansion will be sometime in Q1 next year, that's already confirmed. hopefully earlier in Q1 like Feb. But basically, sometime between Jan-March is the expected release window.
Again, this is all based on the small bit of Info CDPR have given us and could at any point change (I hope it doesn't). But as of right now no official dates on anything.