Hi Johan,
Autoclosing is an advertiser setting that can be set at the creative level, but AdMob's interstitial templates don't set this, so only third party creatives could possibly choose to set this. There is no method of automatically closing the ad using the SDK.
I'd argue that if users were bothered, they'd click the close button before a few seconds. But if the ad was engaging, they wouldn't want the ad to automatically close. To avoid bothering users too much, I would recommend simply limiting the use of interstitials (show only after every 3-5 levels instead of every level for example).
Thanks,
Eric