Not a baker myself but i find yr query a bit confusing regarding objectives/data presented. There is also maybe some thread confusion over terminologies.?
(a) Bakery products may surely contain a variety of allergenic components other than egg/milk?
(b) TBH I thought allergen testing for "sanitation" purposes is more often a routine daily "exercise"
Regardless, i think you are now talking about revalidation of a "Procedure", SOP, whatever. Or, equivalently, a revision of a risk assessment/management etc. I can suggest a basic, generic, approach since I'm unaware of any possible local regulations which might come into play if present.
The frequency is often initially "maximised" for conservatism and then relaxed if no problems. The degree of relaxation depends on, for example, the results/process/product but is usually, and especially, also a compromise in respect to the potential consequences in the event of a significant deviation occurring, eg product retest/recall.
So IMO the basic answer to yr query is to repeat yr initial validation where you demonstrated the current procedure is "valid" then simply, similarly justify the decreased frequency based on additional results/consequences,etc.
Rgds / Charles.C