DC has confirmed The Green Lantern #12 will be the duo's "finale" on that book - but according to the artist, the plan is for the two to reunite for another run in the near future. Sharp revealed the plans Wednesday in the wake of the Green Lantern: Blackstars announcement - a three-issue series starting in November.
"All three covers link up into one sprawling piece, and I’ll be back for at least ten more issues with Grant toon [sic], but this mini-series will rock! It was the plan all along..." Sharp tweeted.
Morrison and Sharp have previously described The Green Lantern #1 - #12 as "year one" or their "first season", so presumably this would mean a second 'season' is in the works - either resuming with The Green Lantern #13 or possibly a new #1.
Green Lantern: Blackstars is projected to run through January 2020.