The goal is to build Meteor to its full potential both as a technology (including open source) and a company. If that happens, the long-run return will be fine.
Failing that, they're hoping that we can replicate the success of JBoss ($420M to Red Hat), XenSource ($500M to Citrix), or SpringSource ($420M to VMware), all of which took open source software and sold it to the enterprise, and each of which did it under the direction of one of Meteor's new advisors (David Skok, Peter Levine, and Rod Johnson respectively.)
As for time frame, they are patient and expect it to take a long time. They know that it takes years for a platform to be adopted.
(edit) I am honestly surprised to see a VC invest into (what amounts to) a fundamental technology. Though, given the valuation bubble we are in, it makes sense. VCs now have a choice between continuing to speculate by funding upload widget startups ($2-3M a pop, two years of runway) and putting money into the core tech. Fingers crossed, this becomes a trend.