How long someone can keep concentrated?

[With a box]

Can he maintain his silent image for a whole day? Or up to minutes/CL?

[Necroticplague]

Until he fails a Concentration check or fails to take the required Standard Action to concentrate on the spell. So while he (whoever "he" is) could technically could keep a Silent Image going the whole day, you'd be pretty useless, since your standard is what does almost everything. Plus, looking under the Concentration skill, their's quite a long list of things that can force a check, including:being buffed, taking damage, and attempt to get around the slower speed from only having a move action a round, crappy weather, being entangled. And if he goes to sleep (presumably because he has to refresh his spells), then it'll disappear (since concentration definitely counts as something mentally strenuous, even if he doesn't sleep, he still can't do it while refreshing his spells).

EDIT:Do, however, note that many of the more useful concentration spells, also have limits on their duration. An example would be Telekineses, which has Duration:Concentration, up to 1round/cl.

[With a box]

Another question :
If I cast minor image, can I cast a spell (and cause stop concentrating) and keep minor image active by concentrat next round?

[Pilo]

No, and even if you could, you have to spend a standard action every round so the spell you cast while concentrating on minor image had to be cast on an other action (swift action, immediate action or move action).
However, there is the Swift Concentration skill trick in complete scoundrel which allows you to concentrate on a spell as a swift action.

[Necroticplague]

Another question :
If I cast minor image, can I cast a spell (and cause stop concentrating) and keep minor image active by concentrat next round?

No. If you ever cease concentrating on a spell with a duration of concentration, its done for. Without the assistance of certain other magics, that's it.

Despite what the dude above me says, even if you have the action economy, you can't cast another spell while concentrating, because of this:


You can’t cast a spell while concentrating on another one. Sometimes a spell lasts for a short time after you cease concentrating.

If course, there are feats to fix this, but by default, you can't even cast swift action spells while concentrating.

[With a box]

But minor image is con+2 round duration

[Pilo]

Lost empire of Faerun as a feat called "Familiar concentration" which allows you to give a casted concentration spell to your familiar under some circumstances.

[Necroticplague]

But minor image is con+2 round duration

And there's no rule that says you can start re-concentrating on a spell after you cease concentrating on it. So the image would last for two more rounds, but you couldn't make it last longer.

[Psyren]

Once you stop concentrating, your spell is "dying" so to speak. You can't simply pick up where you left off, even if the spell is allowed to stick around for a bit.

[Ettina]

Are there rules for Concentration getting harder if you've been doing it too long? I mean, if you've spent several hours keeping a silent image up, it should be getting harder and harder to concentrate on it. Especially once you get to the point where you'd normally be resting for the night.

[Psyren]

There aren't any by default (since concentration spells simply aren't assumed to last very long) but you could easily implement some kind of fatigue houserule if your players want to walk around for minutes/hours with their brows furrowed.

[Spore]

I would use the rules given for hot or cold environments. This is extraordinarily strenous. But substitute the save for a will save. How else is a 9th level cleric keeping up the chanting for a Hallow/Unhallow spell?

[Necroticplague]

I'd modify the overland travel rules. They assume one action a round to move, for eight hours straight. Past those 8 hours, you start taking nonlethal damage for a forced march (since you're still taking an action every round). If you want to move and concentrate both over a long distance, consider that to be a hustle while moving at walking movement rate, and thus start taking nonlethal damage after an hour. If you kept going, you'd eventually knock yourself out.

[Gnome Alone]

Are there rules for Concentration getting harder if you've been doing it too long? I mean, if you've spent several hours keeping a silent image up, it should be getting harder and harder to concentrate on it. Especially once you get to the point where you'd normally be resting for the night.

Fair enough about the resting for the night bit, but actually I could also see it getting pretty easy to keep concentrating on the silent image spell; you've already been doing it for hours, could end up being kind of a zen thing.