Tree pollen can start as early as mid-January, if the weather's right for it, so depending on the trees you react to, your hayfever can start then.
My hayfever usually starts the 2nd week in February - it did this year - and keeps going until end April/mid May.
If I nip it in the bud with a full week of anti-histamines right at the beginning, it usually stays mainly 'at bay' with on or two short flare-ups which I can deal with with a nasal spray. But if it's a bad flare-up I go back on the anti-histamines for another week. Not had to do that for a few years, though, and wearing a mask DEFINITELY helps when out and about in pollen-y areas.