Ok,
I've been thinking about this, and I really kind of liked the 1 minute/level special summon because it allowed the summoner to do non-combat summoning, something no class can currently do. The more I thought about it, the more I realized I'd rather have a summoner who had an eidelon and the ability to use summoning for non-combat purposes. Here's my alternate version of the ability, geared toward non-combat uses.
Enhanced Summoning
A summoner's skilled training allows him to summon creatures that stay with him a vastly longer time than other summoners. At 1st level, a summoner can summon any single monster off the summon monster I list. This creature appears and obeys the summoner and stays for one hour per level of summoner. The summoner, when using this ability, cannot summon any other creatures (except his eidelon), even using a summon spell (the drain of keeping the creature with him disrupts his summon spells).
At 4th level, and every four levels thereafter, the number of creatures he can summon is increased by 2. Thus a 4th level summoner could summon 3 creatures from the Summon Monster I list and keep them with him for 4 hours, but not be able to summon any other creatures (other than his Eidelon) during that time.
At 8th level, the summoner can summon creatures from either the Summon Monster I list, or the Summon Monster II list. He may mix and match as he needs. At 16th level, he may add the creatures from Summon Monster III to the mix.
This is a spell like ability which can be used a number of times per day equal to 3+ the summoners Charisma Modifier. Using this ability takes one minute per creature to be summoned.
---------------------------------
My reasoning :
The only place it is really close to being overpowered is at very low levels, but even then, I don't think it is. None of the 1st level summons are very powerful, the best you might get is really a one hour riding summon (dog or pony if you are small). For combat purposes, the eagle or dire rat isn't too bad, but they are very squishy even at 1st level. However, having a pony to haul your stuff around for 7 hours a day (3 + Charisma of 4) is very nice at 1st level, and at 2nd level, you can do a full 8 hours of pony.
By the time you can summon two eagles or dire rats, they are useless in a fight against anything other than goblins. But you can now summon two pony's or riding dogs. At 8th, where you get access to second level summons, they are all so week that they won't help in a fight, they will just get squished very quickly (possibly in one round with a cleave). However, being able to pop 5 horses for 8 hours at a time can be a lifesaver, or 4 horses and a wolf to scent out a trail you are looking for? Or 5 small elementals to build up a wall (or tear one down if it's undefended), again, lots of non-combat uses.
The most powerful thing you could get would be 11 lantern archon's, at level 20 (or 9 at 16th level). And.. honestly, at 16th level, 9 lantern archons are just so much toejam to a BBEG. A single area effect spell will take them out. Even grunts at that level will take them down in one hit. The big thing is, you can use that 9 creatures at 16 to summon up 9 archons and have them help you rebuild a village, or dig a water well, lots of non-combat stuff that would really make a summoner popular and give him a niche.
Hmm I don't know. On one hand it seems a bit strong to have basically two pets running around constently. On the other hand it makes the summon spells almost pointless unless the summoned mobs die fairly often.
Yep, the idea is that the long lasting pets are really very weak in a fight, so it's not a balance changing thing. It's a RP and utility thing. As I said above, popping out 4 horses and a wolf to do your tracking at 8th level is a very good non-combat ability, but almost useless in combat, as a horse or wolf dies almost instantly in combat at that level.
Yeah it is likely fairly balanced cause I can't decided if it makes the class to good or gimps their spells to much.
LOL, that usually does tend to indicate a balanced ability. :)
The only really bad I think is that it does remove a lot of the use of their summon spells. Might need to add a few more spells maybe or loosen it up to no more than 1 SU summon and 1 spell summon at a time each or something like that.
Well, he can always dismiss the long term summon to summon up some other monsters for a temporary thing (like a fight), then resummon using his SLA afterwards. He can dismiss the SLA at any time as a free action. It just takes a turn to cast his normal summon spell from his spell list, just like any other caster. He just can't keep his long term pets around while doing it.