When will these allergies or cold stop acting up?

On Sunday I thought I was coming down with a cold. I developed the typical clogged sinuses, sore throat, and cough I usually get. Here’s the kicker: when I get a cold it starts out with a pretty weak case of the clogged sinuses and sore throat and then migrates into a nasty chest cold that takes a while to get over. I knew something was up when when the sore throat stuck and got pretty annoying – that and the clogged sinuses were here to stay as well. A quick trip to the doctor and I was told that I had some kind of nasty allergic reaction which clogged my sinuses, gave me a post nasal drip which in turn caused the sore throat and asthma to act up. She prescribed Astelin, a nasal spray antihistamine and described the allergic reaction as a sort of systemic thing and that the astelin should clear it all up. That was Wednesday. I’ve been using it as told since then but only noticed the slightest improvement. I’m wondering if this is normal and it’s gonna take a bit for this to clear up?