How much does an allergy change over time?

Depends. Allergies are like baskin robbins, they come in 31 different flavors. People become less allergic over time but the symptoms can progress or even remain despite negative testing.
Quite a bit! Both food and environmental allergies can change over time. I have many patients who were allergy-free during childhood, only to develop hay fever as adults. The same may happen with food allergies. On the contrary, some patients "outgrow" their environmental or food allergies, and do much better as adults. So the natural history of allergies can vary quite a bit!