The single most useful thing you can do for a visa application is start it earlier than you think you need to. The cost of being early is mild inconvenience; the cost of being late is a missed trip.
Below are the windows that advisors at Axis Visa recommend for the 19 destinations the service supports. These are conservative estimates that account for normal seasonal queues, not worst-case scenarios.
Digital travel authorisations (ESTA, eTA, NZeTA, Kenya eTA).
Earliest you can apply: as soon as you have flights confirmed. Latest you should apply: 72 hours before travel for ESTA, 48 hours for the others. Most are approved within 24 hours; the safety buffer accounts for the small percentage that go to manual review.
Tourist eVisas (India, Brazil, Sri Lanka, Kenya).
Earliest: 30 days before travel for India (validity window starts on the visa date), as soon as you have flights for the rest. Latest: two weeks before travel. eVisa systems sometimes go offline for maintenance windows that can stretch a routine application by several days.
Consular visas (USA B1/B2, Canada Visitor Visa, Schengen).
Earliest: three to six months before travel, depending on consulate. Latest: depends entirely on appointment availability, which varies enormously by post. London Schengen appointments in March are not the same lead time as Lagos Schengen appointments in March. Six weeks is a safe planning minimum; twelve is comfortable.
China L visa.
Earliest: 90 days before intended entry (validity counts from the issue date). Latest: at least four weeks before travel, longer if you live outside a major city and need to ship the passport to a CVASC.
Axis Visa is not a government organisation. Lead times reflect advisor experience and change with seasonal demand.