15-20 minutes. Not every movie has to be a 2 hour origin story (despite what many early comic book movies would have us believe). Look at 2008's Hulk. It was an unofficial sequel but it took care of the whole gamma accident, military-conflict backstory during the credits. Sure Hulk may be a household name and Pym is unknown to anyone who doesn't frequent comic shops but a charismatic actor could win audiences in less than a half hour. Television sitcoms do it all the time with pilots - establishing major concepts and themes in very little time.
JARVIS never sat well with me in the MCU. The concept of an intelligent AI helping to operate the suit largely originated in the first Iron Man movie and was shoe-horned into the comics 5 years ago in an (unfortunate) case of reverse influence (a problem that is growing because of movie popularity). While I found many, many problems with Iron Man 3, JARVIS was amongst the least of them. However, between the Mandarin, Extremis affecting soldiers and Stark, and multiple armors, JARVIS was an unnecessary addition to the plot that was far more of film-maker shoehorn than source material influence.
In short, and this is just an opinion,making Stark responsible for Ultron gives the character way too much influence while JARVIS seems like a crutch that actually casts a poor light on Tony's abilities. Comic book Tony Stark never relied on an AI to operate the Iron Man armor(s) to such an extent. (Extremis -enhanced comic Tony piloted 40+ suits directly via mind link, not having an AI act autonomously).
Again, these are all just opinion statements.
EDIT: And where better to establish a character who's most defining moments were presented in the pages of the Avengers (starting with issue #1 in 1963!) than in the Avengers movie itself. By using each standalone movie to build to a plot-light Avengers film every three years, the quality of individual films decreases (see: Iron Man 2). Each film should really stand alone. This will get me flack, but Hulk does not need to cameo in Iron Man and Hawkeye did not need to be in Thor. Characters crossover all the time in the comics world but the best stories usually focus on the title characters alone (obvious "side-kick" exceptions aside - War Machine, Falcon, etc.). The Avengers films are really the best places for franchise interactions.
EDIT #2: Before someone comments, Tony has had problems with AI in the past but it was NEVER a major plot theme prior to the MCU. I'm not pissed about the creation of JARVIS. That's an addition to the film franchise. Not having Pym involved subtracts from the Avengers legacy.