Improvement can be quick, or it may take years to overcome the problem. The reason spring allergy is often very difficult to overcome, even with homeopathy, has to do with miasms, which are inherited weaknesses that we come in with. There are at least 3 different miasms, and if you have all of them, and they are really tangled up in each other, you become susceptible to spring allergy, or many other allergies for that matter.
So when dealing with very complicated cases, the acute symptoms have to be addressed first, and then later, when allergy season is over, deeper, constitutional, underlying weaknesses have to be addressed, and slowly, slowly, layers upon layers of old inherited stuff have to be "peeled off". This can be a long process, and you may have to put up with some amount of discomfort during allergy season while the process is happening, since you can't use homeopathy and suppressive allopathic medication at the same time.
It is important, however, to look at what the real benefits are in the long run. Allopathic medicine offers short term relief for spring allergy, and long term worsening, since any suppression leads to deeper lying problems later. Homeopathy offers long term healing, even though the short term symptoms of your spring allergy may be bothersome during the process. So you have to decide what is more important to you - short term relief, or long term healing.
Sometimes the correct remedy that works for the acute symptoms is also the constitutional remedy 9fitting the person on all three levesl) and whenever that happens, the healing happens quickly.
But, for some, the use of homeopathy for spring allergy is a lot more complicated. Personally, I feel that it is still worth choosing homeopathy over traditional methods because the use of homeopathy on a regular basis will eventually make the body stronger and healthier since there is no suppression of symptoms involved.