It's typically been about 3.5 years between iterations
PSR3000 late 2004 (based off Tyros 1, really cut into sales of T1, Yamaha learned their lesson)
S900 late 2007 (between T1 and T2; same chipset as CVP405)
s910 late 2009 (based of T2, but T2 strings and brass in mono; same chipset as CVP505)
s950 late 2012 (T3 with no SArt2, but added mono versions of the T4 drums. Same chip as cvp605, but with Audiostyles thrown in)
s970/975(same cpu) 2015 -- the s975 was not actually a new model; it was the s970 but with more user space, and 4 packs preinstalled. But I get how it makes it look like there's a new board every 3 years. Based off: T5 drums, T3 chipset but with with the T4 ensemble strings and brass thrown in. Or you can look at it as a T4 with most of the choir, all the solo strings, and solo brass, and all the Sart2 sounds removed, but with AudioStyles thrown in
sx900 late 2019 (essentially a T5 with no Sart2, but with live versions of Revo drums)
Fred is correct, they generally alternate with new flagship models first (Tyros/Genos) and then trickle down.
Tyros 1 late 2002
Tyros 2 late 2004
T3 late 2008
T4 late 2010
T5 late 2013
G1(T6) late 2017
Considering how expandable Genos is, yes it should have a much longer lifespan than any previous Tyros. Which also increases the gap time for new PSR; but they can always pull a s970/s975 where they release the same keyboard with a few more goodies thrown in.
How many PSR's can they squeeze in between the Tyros models depends on the 'jump' between them:
Tyros 1 was a big jump from PSR9000
T2 was a big jump as well; T3 added SArt 2 to T2 (and new drum kits) but without the SArt2, the changes were small
T4 was a huge jump from T3, but T5 was a small jump from T4; again mostly just a smattering of new Sa2 sounds, a few Sart solo string sounds, Audio Styles, and for the 1st time the option of 76 keys. However, making it work with Expansion Manager was a big deal.
G1 was again a large jump from T5
Mark
PJD could go into even more detail!
Mark