El Chaltén is a summer destination. The hiking season runs from October to April, with the peak months being December, January, and February when days are long and temperatures are mild (5–20°C). Outside of this window, most businesses close for the austral winter, trails may be snow-covered, and the weather is genuinely dangerous.
The problem with Patagonian weather is that it is unpredictable at any time of year. Multi-day clear spells do occur in summer, but you can also have three consecutive days of howling wind, horizontal rain, and zero visibility. A 10-day stay gives you a realistic chance of seeing at least one clear morning on Fitz Roy. A 3-day stay is a gamble.
November and March are good compromise months: fewer crowds than peak summer, still manageable weather, and lighter gear requirements than mid-winter. School holiday periods in Argentina (January, July) bring more domestic visitors; book accommodation in advance.
The wind is the defining element of any visit. It typically picks up in the afternoon and can be strong enough to knock you off your feet on exposed ridges. Plan to start hikes early.