What is the DM stance on taking spell focus (and GSF) in a useful school (like enchantment) but not taking it in your chosen school?

Does SF and GSF in conjuration do anything in particular, like extend the amount of time that your summoned creature will stick around or its toughness?

If you're a specialist wizard, you would want to have Spell Focus or Greater Spell Focus in your feat list I would think. Especially so, if you have other Spell Focus feats for different schools other than the one you specialize in!

Spell Focus will give you +2 DC, and GSF will give you a further +2 DC on respective spells of that school.

Spell Focus in Conjuation will also benefit you with extended duration of summons. Exactly how much longer is best found out IG.

Thank you!

Personally, I find devoting yourself to a particular school X and then taking Spell Focus : Y pretty lame, as usually school X is taken for the extra spell slot and to not loose "good" spells in the opposing school, and Spell Focus Y is taken to augment what you really want.

It's been discussed before.

Along the same lines as conjuration focus extending the length of time that summoned creatures can stick around, would spell focus: transmutation or abjuration have any influence on the length of time that buffs will stay up or how potent they are?

I believe Abjuration augments your dispels and the base bonus, but the benefits of Transmutation are only tangible for spells that affect others negatively(Curse, Flesh to Stone).