How much does a wife inherit under Islamic law?

Agreed. The surviving husband takes one half of his wife’s estate if she left no child or son’s child, and one quarter if she did. The surviving wife takes one quarter if her husband left no child or son’s child, and one eighth if he did. Where a man leaves more than one widow, they divide that single quarter or eighth between them — Ibn Qudama puts it as “one or four, it is the same”. He records the whole of this as the consensus of the people of knowledge, and Ibn Rushd likewise opens with “the jurists agreed”; both treat Surah an-Nisa 4:12 as settling it outright. Two features of the spouse’s share follow from the same verse and are worth stating: nothing but a child or a son’s child can reduce it, and nothing can exclude a spouse altogether — a husband or wife is never shut out by another heir. We checked this point; the schools do not differ on it.

Shared evidence: Surah an-Nisa 4:12 — the half, the quarter and the eighth, stated expressly.

Not covered by this finding: how a spouse’s Qur’anic fraction interacts with Canadian matrimonial property law — an equalisation payment or a spousal election is a claim against the estate before the shares are worked out, not a share in it — and whether a surviving spouse takes anything in the surplus where no other heir exists (see radd-spouse) — not yet researched school by school. Treat that as open and ask a scholar of your school.