Validity periods follow a pretty standard menu worldwide: daily packs, 7-day packs aimed at tourists, 28 or 30-day packs for longer stays, and sometimes 15-day options in tourist-heavy countries. Read the fine print on what happens at expiry, because some plans just stop, some auto-renew and eat your credit, and some let unused data roll over if you top up before the deadline.
Topping up has become genuinely easy in most places. The carrier's own app is usually the smoothest route once you register, though some apps need a local payment card, in which case their websites often accept international cards. Beyond that, convenience stores are the universal fallback; you either buy a scratch-style voucher code or the cashier tops you up directly by phone number. Many countries also have top-up kiosks or machines in metro stations and malls.
One habit worth adopting: screenshot your SIM's phone number and the carrier's USSD balance-check code the day you buy it.