How long between each use of an agent, am i allowed to use another on it

If you mean Brewing Agent..I'm not sure how many minutes before it's condition goes bad, and yes you can use an infinite amount of agents on it.

It's about an hour

the brew rollover timer is ~35-ish minutes. I can tell you this because I use a timer to keep track of when we need to add our next brewing agent, and that's the time that works- set it for just 30 minutes and you're stuck waiting around for a few minutes.
However, you get... an hour, I believe? after it starts needing a new additive before it loses a quality stage

Oh, good to know! I always thought it was an hour, is it for other fests?

timers are always between 35 minutes and like. 1hr7minutes I think was the highest one I've seen? Depending on the fest. don't ask me which fest has how much, I do not retain the info

Generally, I keep a timer on hand, when I start my first growing I set it for 45m, and when the timer goes off, see if it needs attention. If it does, drop timer to 35 and repeat, but if it isn't done, keep an eye on the time to work out when it does roll over to needing attention. And then I can just keep the timer set up to go off when it's time to put additives in