It depends very much on the accent. American and Australasian accents use [ə] in a lot more positions than RP British does. In RP, as a rough guide, [ɪ] is used whenever the spelling has E or I: all the unstressed vowels of decidedness or indivisibility, for example. It's also used in some final endings containing A, such as in damage, palace. But in American and Australasian accents these are usually all [ə]: they use [ɪ] in a small number of unstressed endings; one common one is -ing. Southern BrE is using [ɪ] less and less these days, becoming more like AmE and AusE in its choices.