My pollen allergies in the past have been quite intense, so I have good experience. I've been basically forced to be inside a house with the AC on(if we had it, which we didn't for quite a few years) and then having to down a lot of benadryl to knock me out because it's horrible.
So, yes. It can go anywhere from just a few seconds outside for a full blown reaction to maybe hours before it sets in. I don't like to risk it if I can avoid it. It's usually why I hate summer the most out of every season. It's great outside, but I am stuck inside because there is nothing I can do out there when I have an allergic reaction. The most incredible itchy eyes, Niagara falls of runny noses that never ends, constant sneezing, and headaches and tiredness from these side-effects... it just adds up to hell. It doesn't sound like much, but it's horrible. It's not like, *itch* ahh, fine. It's like *itch with sandpaper* "WHY WON'T IT STOP; GOD IT'S SO HORRIBLE!!!! D': AHHH, MY EYES HURT I CAN'T SEE AND IT'S AAAAHHHH!!!!" Runny noses that basically flow down your whole face dripping you in a combination of water and snot that are impossible to contain. It's just horrible... can't do anything and it's horribly irritating.
I don't usually break out in rashes and some more severe stuff like some people do though.