This is how I play it. This is 4e, but maybe heavily modified to my taste and not the "proper" way.
Perception to notice something new
In this case, I'd say just one roll. If the DM think it's something that somebody needs to find out, he may give you a second opportunity to pick it up (e.g. nobody notices the thieves' guild tattoo on the wrist of the merchant you just met, so he has the merchant reach for something off a high shelf, giving a second chance for you to see the tattoo as his sleeve pulls back). You shouldn't get infinite chances. At the DM's discretion, he can give multiple chances. If the DM thinks the party absolutely HAS to know something, he can have a roll and if nobody beats the DC, he just gives a hint to the person with the highest roll (e.g. "Terris, as he reaches for the book you notice something that makes you feel very uneasy about this person, but your concentration breaks before you pin down exactly what you saw.")
Perception to find something you know is there
If you have unlimited time/no pressure, this roll can just tell you how long it takes you to find this. If you are under pressure, this can tell you whether or not you find it "in time" or "this turn", as appropriate for the situation. Looking for the hidden switch to open the door in combat, for example, would be a standard action and repeatable. Looking for the switch before the kobolds find the party would be one roll (per character) to see if they find it in time. Looking for the switch after the kobolds have been defeated is just a roll to see how long it takes.
Beyond these, the DM should modify DCs to how difficult the task is. Finding the switch in combat will be difficult, as the character will probably be occupied with dodging crossbow bolts and watching their friends fight for their lives. Finding the switch before the kobolds get there is probably easier, but the characters are probably still eyeing the door and readying spells and looking to make sure there isn't anything coming down the other hall. After combat should be easiest, as the characters have few(er) (life-threatening) distractions.