According to Baseball Reference: the league average for starts is 5.4 innings.
https://www.baseball-reference.com/leagues/MLB/2018-starter-pitching.shtml
Average innings per start for last year's "incumbent" starters:
Hess: 5.3
Bundy: 5.5
Cobb: 5.4
Cashner: 5.5
FYI, Straily was 5.3 last year and if you stretch it out to hundreds he was lower than Hess.
The guys we have are fine, especially if you're not concerned with winning this year. If it's an innings eater you want, there are probably better options than Straily for 5 million per year.