Nice catch on the ghazneths!
A minute of draining sounds decent.
If we want to let it drain permanent magical items, why not just use the same formula as the ghazneths?
So, something like this?
It also seems we need something like this:
Dampen Magic (Su): Magical leeches reduce the efficacy of magic. Any spellcaster within 3 ft of a magical leech must make a DC X caster level check when attempting to cast a spell. If the check fails, the spell does not function, but the prepared spell or spell slot is lost.
Well, that might be a bit fiddly. I could definitely see adding a save and possibly requiring longer contact (like a day or more).
Could go the SR route, too. Works about the same way, I guess, and it's a little more elegant. Sure.
Dampen Magic (Su): Any spell cast within 3 feet of a magical leech is affected by the leech's spell resistance, even if SR does not normally apply to that spell. If multiple leeches are within 3 ft, only the highest SR applies.
How about each day they're in contact with a magic item, the item makes a save, with the DC increasing by +1 each additional day of contact with a given leech?
Absorb Magic (Su): A magical leech can absorb spells from nearby spellcasters. For each minute that a spellcaster spends within 3 ft of a magical leach, the caster must make a DC X Will save. On a failed save, the caster loses one prepared spell or spell slot of the highest level remaining, and the leech absorbs a corresponding number of spell levels.
In addition, a magical leech can absorb charges from charged magical items, absorbing one charge per minute of contact. A charge grants the leech as many levels as appropriate to the effect (for example, a charge from a wand of magic missile gives 1 level, but a charge from a wand of fireball gives 3 levels). Charges of non-spell effects grant one level for every two caster levels of the effect (rounded up). The charges are forever lost to the magic item, as if they had been used normally.
Magical leeches may also spend a day or more to discharge a permanent magical item (or one with daily charges) completely, rendering it a mundane item. This ability requires continuous contact. The leech gains 10 spell levels per spell level of each effect of the magic item (or 1/2 the caster level of the effect, rounded up). Each day of contact with the leech, the magic item has a DC X Will save to negate the drain, but the DC increases by +1 for each day beyond the first that the leech maintains continual contact.