... you made Bull's Strength, Cat's Grace, etc round-based and 1 min/level?
You could "turn it off" with a command word at the end of the fight (or whenever you wish) and activate the remaining time when needed?
That would make it so it was basically lasting all day as you would have it whenever you needed it. For that I'd increase the spell level by two and make it a standard action to turn on and off.
Might as well make it 24 hours - there just aren't that many combat rounds in the day, and physical stats are only important then, for the most part. I could see Communal versions of those spells like in Ultimat Combat, but doing what you suggest will be pretty close to a flat +4 bonus to important stats. You'd have to increase the spell level to compensate.
If you hew to the on/off switch and rounds per level be prepared for additional record keeping which could be annoying or have a detrimental affect on your game.
And they changed it in 3.5 for reasons.
At 1 hour/level it lasted far too long and d4+1 meant that meta-magic feats like maximize and empower could be used on it (and I believe that empower could be stacked...)
Moment of Strength
3rd level spell
M/DF, V, S
Touch
+4 to Str
The effect lasts on the person for 1 hour for every 2 caster levels (Maximum 5 hours)
As a swift action you can call upon the strength. this effect lasts for 1 round per level or until suppressed (a move action). If suppressed before it would expire you may reactivate it later (another swift action)
in this way, you have it available for a few hours but it wont be all day
And they changed it in 3.5 for reasons.
At 1 hour/level it lasted far too long and d4+1 meant that meta-magic feats like maximize and empower could be used on it (and I believe that empower could be stacked...)
Making it on demand is extra work to keep track of and kind of silly. I'd just go for a longer duration. You could up the level, but I never saw the 1 hour/level 3.0 versions cause any problems. It doesn't increase spells per day, and using a 2nd level spell to grant your fighter +2 attack and damage is hardly gamebreaking. Also, nonstacking ability items are ubiquitous.
I seem to recall the argument for switching from 1/hour to 1/min per level in 3.5 was because many groups were casting these sets of spells every morning, instead of going with permanent magical items.
It never really bothered me much, since the spells were easier to disrupt with Dispels, and it freed up item slots for other, more flavourful things.