Mei Mei,
I don't think there's any pat answer on how long fatique lasts. For me personally, my sleep patterns seem to be forever changed. Before my AN surgery, I could stay up until 1-2 or 3am each night and be up getting ready for work by 7-8am.. No sweat. After my AN surgery, while I can stay up late if I need to, I'd rather go to bed earlier and if I don't get a full 8 hours of sleep, then I'm cranky/tired/both the next day... Would you call that fatique? Dunno, but things are different now with regard to my sleep.
All you can do really is push yourself a little and slowly start getting your stamina back. And learn to adjust to the new 'normal' as far as your required rest goes.
It's very likely that the added pollen in the air could be affecting your fatigue level. I'm not sure I'd say it was causing an allergic reaction (unless you had allergies prior to AN), but it tends to tax our systems as a whole more during those times (or at least it can) and we feel wonky head more, headaches more, fatigued more, etc...
I'm almost 2 years out and things are still not really back to what they were before my AN surgery..
Regards,
Brian