When the Whisperer plotline ends is hard to say as Kirkman starts blending the next story with the end of the Whisperer one. I'd say they last about 40-45 issues, so it took over 3 years to play out. though the plot's end in the comics is a little underwhelming, and I think they can do it better on the show than they can in the comics. To compare it, the entire Negan plot was about 30 issues, and they released All Out War on an accelerated (12 issues in 6 months) so that story took just under 3 years. Funny thing is that they stretched out the Negan/Savior story to nearly two and a half seasons on the TV show, and The Whisperer Arc, which is much longer, and a slower burn which needs to be told a little slower, will probably wrap up by the end of Season 10, though it might bleed into 11.
They've actually already started planting seeds for the next story as well. They started last season, actually. They were planting seeds for that before they did their Whisperer tease in the Season 8 finale, which made some people think they might skip The Whisperers altogether...which might have been enough for me to stop watching the show, since I love this story so much. A lot of Walking Dead comics readers adore this story as well.
As for what happened between the communities, in the preview for the next episode there is a shot with old looking Michonne...not sure if it's between 8/9 in a flashback, or if it's even further back than that. I have a feeling they will show some flashbacks over what happened over the course of the back half of this season, which would make narrative sense. It's strange this new take, because in the comics, all of the communities are working together and living in harmony, and the fair is happening at Alexandria and is meant to be a reward for progress. Now, with the communities apart, it seems that the fair is Ezekiel's attempt to bring the communities back together again. And to make things even stranger, the only group that wasn't fully onboard with the other communities were The Saviors, who felt like outsiders, at The Sanctuary...but in the show The Sanctuary isn't even around anymore. A big part of The Whisperer story is The Saviors reluctance to help the other communities out with The Whisperers.