Domestic mail speed varies a lot by country, but the general shape is consistent. Within the same city or region, standard letters usually arrive in one to three working days where postal systems function well; between distant cities, two to five working days is typical, with remote or island areas adding more.
Tracking is the real differentiator. Ordinary stamped letters are untracked almost everywhere; if it matters, pay for registered or recorded mail, which adds a tracking number and a signature at delivery. Parcels through the postal service are usually tracked by default now, and domestic couriers in many countries offer same-day or next-day city delivery with live tracking at modest cost.
Practical habits: never send documents you cannot replace by ordinary mail, allow extra days around public holidays when backlogs form, and for anything genuinely urgent domestically, a courier or even the intercity bus companies' parcel services in some countries beat the post comfortably.