In the case of Aid Alchemy (or Sorcery, or Study), a single service would assist a single preparation (or spell, or learning). It doesn't list a time because each of those things can be different amounts of time... (Ritual Spellcasting time is variable, and Learning is days based on the test's result)
Simply having a spirit bound doesn't mean you get an infinite amount of assistance for just one service.
While I'd understand that ruling, I think it's ridiculously expensive given how it costs a Bound Service (which is easily 500 nuyen if you're not real good and rolling dice) and the language (Tests, not Test) doesn't support it. Given how the other bound services explicitly note a duration, there is nothing solid here regarding the duration of this service. And I'd find it strange if 1 service could cover anywhere from 1 Initiative Pass to weeks, depending on the kind of aid.
Then what's the limit? Use one service and now the bound spirit aids every test you ever make?
It uses the language "Tests" (plural) because the subject is multiple types of tests (Alchemy, Sorcery, or Study). Grammatically, because the subject has multiple parts, the verb must be plural, even if it is only referring to a single instance. F.e. "On any one of your Math, Science, or Reading Tests"
And like you said, each of the other types of services give an explicit duration, which means this type of service defaults back to the basic definition of a service, which is a single use of an ability. You ask it to aid you on a given spell, it does, and the service is done. There is no language listing a time, or a number of tests, therefore there is nothing to imply that you get multiple "Aids".
