Don't think you can set any hard and fast rule for how many years a guy must coach before he gets a head coaching gig. Everyone is different, and it doesn't make sense to limit your pool of candidates to an arbitrary rule like that. Like Manning, Kevin Ollie spent most of his post-college career as an NBA player, and he was even an assistant as long as Manning, never had been a head coach, and Ollie won a NC at UCONN. Similarly, Chris Mack was an assistant at WF (of all places), got the Xavier job, and has won 20+ games there each year, but one over his 7 years there. Andy Enfield (who many people here wanted WF to hire) coached Florida Gulf Coast, a level clearly below Tulsa, for only two years before getting the USC job, and they now look to be a tournament team. Will Wade, who has done very well at VCU replacing Shaka was head coach at Chattanooga for only two years before getting the VCU job. Tony Bennett coached Washington State (his first job; so Wazzu hired him with no head coaching experience) for 3 years before getting the UVA job.
Manning had been working under Bill Self at Kansas for 9 years before he got the Tulsa job. So, he has been coaching college hoop for 11 years before taking the WF job. If he fails, don't see lack of experience as a factor. Totally understand the frustration with how this team has fallen apart as the season has gone on, but good coaches have had seasons like this. While I am more skeptical now than I was a couple of months ago, still think Manning needs time and patience before a conclusion can be made whether he was the right hire or not.