Most likely never. Wifi is a shared medium (unlike Ethernet where you have two separate transmit and receive wires). You can't have two signals sent at the same time over the same frequency. The only way would be to split the frequency band in two, leaving you with only half of the bandwidth in either directions.
I agree.
Ethernet also started off as a single cable half duplex shared medium and it took many years to evolve to full duplex. Simply adding the 2nd wire didn't do it as they were still collected at hubs, Layer 1, multi-port repeaters, also half duplex by definition. It wasn't until the advent of switches, multi-multi port bridges at Layer 2 that full duplex became a possibility.
So start with a dual-band (two radios) AP, one for send and the other for receive, and you have "full duplex". But to add another user you need two more radios? Uh, that's not going to scale well.
So instead of using a channel width of 160 you might try 8 channels with a width of 20 each? That's 8 channels, 4 up and 4 down ... four users total. So not only do we have a limited number of users we also have everyone running slower. MU-MIMO? Even if/when MU-MIMO works we run into similar scalability issues.
So, at least for the immediate future, better to go fast and find better ways to share. I'm thinking things like "Airtime Fairness" or 1x1 MU-MIMO clients connecting to 4x4 MU-MIMO services?
EDIT: It occurs to me, while Full Duplex may be nice, how important is it ... really? Many, me anyway, are "leeches", bidirectional communications are minimal. We
download; Movies, books, TV shows, news.
Up communications are typically "ACKs"; "ok, thanks, send me more". Our attention is probably better directed to multiple WiFi users all living in the same collision domain.