Don't think it matters how long you have it done before it is due, as long as you don't go past the due date. Mine was done a month early last year, so mine is now done with most of the yard, so we can share a call out.
Most rules just state the the annual booster must be given within 12 months of previous vac, they do not say it must be given after a certain length of time.
If you are having an annual booster it must be done on or up to 6 months before the same date it was done on last year - there is no lee-way on this for competing under RC rules. (If you compete under FEI rules you have the booster done every 6 months and you have 21 days leeway on the date from last year.) If it is a new course, then you cannot compete until 7 days afer the second injection. However, it is recommended that you do not get your horse sweaty for at least 48hrs after any flu jab as it can sweat the vaccine through the pores of the skin and it will not be covered. Clear as mud?!!?