Looks like most all of them are administered at 8wks and before 4wks of onset of lay.
As for giving a booster, I don't see mention of it in the manufacturer's literature, but I think the general consensus is to give another at 14-16wks of age.
That said, I'm not sure if a booster is needed if you note that the vaccine "took". According to the literature from one maker:
"""EXAMINATION OF TAKES
Examine the site of vaccine administration for “Takes” at 5 - 7 days post-vaccination. A positive “Take” showing that the vaccination was successful, is indicated by swelling or formation of a nodular lesion or scab at the site of inoculation. Swelling and scabs will disappear at about 2 weeks following vaccination. The absence of takes may indicate that the birds were immune before vaccination or that improper vaccination methods were used. Immunity has been shown in studies to develop at 21 days post vaccination."""
Reference
https://valleyvet.cvpservice.com/product/view/1506010
If you are not finding specifics you are looking for, then calling the maker/manufacturer of the vaccine you wish to use should give you more information.
Here's where you can purchase Fowl Pox Vaccines.
https://www.valleyvet.com/ct_detail.html?pgguid=63c9bff7-0ae9-41a5-bce7-773d195ee78c
https://meyerhatchery.com/products/pigeon-pox-fowl-pox-and-avian-encephalomyelitis-vaccination-kit
https://www.rogierpoultrysupplies.com/product-page/ae-fowl-pox-vaccine
https://jefferspet.com/products/fow...H--SFaYwO0He_Ch6yOrvYEFV1crfXoR8aAlAYEALw_wcB