That's probably too strong to be metamagic. Just for example, it would make Fleshworm Infestation a no-save kill spell (or rather, save ninety times or die). It also gives six to ten times the uses of any spell that reduces the duration by an amount when it's used, like Flame Strike.
There are lots of ways other than what you mentioned, but they are less powerful than Extend Spell, so not what you're looking for.
Just speaking in general, a +3 metamagic to get 10x duration would probably be reasonable. Roughly equivalent to using Extend three times in 3.0 (actually that would be just 4x due to multiplier rule, hmm). But Ring of Continuation is probably the largest duration extender around.
Other ways, probably not enough for your purposes, are by increasing caster level:
Simple spell research for a longer duration version would work. Some spells may not (like QuidEst's example).
Sorry in advance if this is the wrong place for this post.
I am looking for ways to extend the duration of spells outside of Extend Spell and Ring of Continuation. So far I have not found anything. Any help would be appreciated.
Do you think that a metamagic feat/rod that bumped up duration to the next category would be acceptable (similar to Reach Spell for distance categories)? I was thinking it would be either +2 or +3 and follow the progression:
Round/level
Min/Level
10 min/level
Hour/levelLet me know your thoughts.
That would contravene the much of the whole point of 3.5 which was to nerf the hours per level attribute buff spells.