This may be too detailed for some, and may give some spoilers, but here goes:
Of all the summon spell type creatures available, all but 1 have short summon times.
The one that does have a decent time, both for adventure and RP is the Selgarns Persistent Blade (SPB) That lasts 1 hour+1 turn per 2 levels.
All the spells can be extended to double lasting time via the extend spell feat. Most casters tend to either take this feat, or use the available gear that grant it.
Some class abilities that summon something- the Cleric Death Domain for instance gives a very short summon when compared to the spells of the same level- the Shadow summoned duration does not change with levels. The shadow lasts for 1 RL minute- with no feat to extend it.
Wizard and Sorcerer Familiars are completely imbalanced to each other- the most powerful being the imps and mephits which have regeneration, in addition to fire, ice, or poison bite damage- the weakest familiars being the cat and bat. In terms of RP it doesn't really mater as the familiar lasts till dismissed, or killed.
Druid and Ranger animal companions are like the familiars, except not one of the choices regenerates as the imps/mephits do.
With SPB, one can solo most of the low level dungeons. By buffing the SPB, with Cats, Ghostly Visage, Shield, Mage Armor, Magic Weapon, and even Haste, and Stone skin, that blade will take out undead up to the dry liches in Har' Akir. It can certainly handle the Knights and Hecuva priests that sometimes appear in the morning lord crypts, and other locations.
With the characters Isi and Rose, we tested the Assassin Imp familiar at 17th level, FvP against Rose (cleric) at 18th. Fully Wizard buffed against fully buffed+ enchanted gear- the imp won every time, in fact the imp never went under 1/2 hit points, and only got that low due to Rose's harm spell.
As a arcane caster with a imp, or mephit, it is possible to solo most of the server, provided that you can make sure to UN-summon it if need be to avoid familiar death.
To balance it out, especially for the sake of RP, I suggest this, Spell summons duration be changed to 1 hour plus 1 hour per 5 levels. (Our in game hour is 6 minutes?) yes this means summons will last significantly longer. Low levels will get a decent summon time of 1 hour. Level 20, will get a 5 hour summon- and probably never use it for other than RP.
Death domain Shadow summon duration to 1 hour plus 1 hour per 4 levels.
** please note that there is no adjustment of the creature stats, or CR just time.
Familiars should be completely adjusted to be more in line with PNP style.
SPB, should have the concentration check re-added that can cause it to UN-summon if the caster attacks, casts a spell, or pick something up.
All Summons, Animal Companions, and Familiars should probably have the AI added to to flee enemies that are more powerful. (this sort of system is available on the vault now.)
At caster level 10, All the summon spells should have a version of the chance for a mad/hostile summon to spawn in (as the summon7,8,9) do.
While it is possible to solo with the current settings, Most of the Main dungeons in the various domains are set up for a full party- so soloing is nearly impossible until a caster gets the Shape change spell. (another time for that one.)