How much XM is generated around a portal when it's left alone for awhile?
A portal generates 10% of it current energy as XM about once every 15-20 minutes.
This means that a high level portal that is almost dead has almost the same XM yield as an unclaimed portal. This mechanic is added to encourage you to recharge your portals!
For example, my desk portal usually has 30k points of Energy. So, I'd get about 3,000 XM every 15-20 minutes.
Of course, this changes if a portal is weak. For example, a nearby portal has 2k points of energy out of it's max 10k. This portal will yield me about 200 XM / 20 minutes.