So GT Sport was released in 2017. GT7 was release in 2022. That's 5 years. For the chronic cell phone users that means we have another 2 years with GT7....give or take.
It's always hard to make a guess on that. It depends heavily on what they are doing. E.g. there was only a 3 year gap between GT5 and GT6, as GT6 was built on the same technical foundation as GT5 (also on the same content with additions).
GT Sport used a different (or heavily modified) technical setup, not at least due to the different architecture of the PS3 hardware.
GT7 also had major changes under the hood.
If they want to extract more from the PS5, without the limitations of the PS4 in GT7, it can be expected that there are again many changes on the technical side for the next installment.
If they want to add much more new content it's the same, the creation of 3D models is taking much, much longer than it did in the past.
Then again, the gaming landscape changed a lot in the past years and there were budget cuts and reduction of staff pretty much everywhere. I don't think it was reported that PD did the same, but that could still be possible.
Long story short, the development time is dependent on so many different things, it's very difficult to make an estimate like this without having any inside knowledge. We could get a GT8 soonish, or we could get one in the PS6 era as well. We don't know.