Here is the duration of major alt seasons by year (approximately, based on key metrics: drop in BTC Dominance, rise in ETH/BTC, rise in altcoin capitalization — Total2).

2017 alt season

• Period: December 2017 — January 2018

• Duration: ~45 days

• Comment: Mass FOMO, many altcoins showed 10-100x.

2021 alt season (main wave)

• Period: March 2021 — May 2021

• Duration: ~60 days

• Comment: Peak capitalization of Total2, ATH of many DeFi, NFT, and meme coins.

2021 alt season (second wave)

• Period: August 2021 — November 2021

• Duration: ~90 days

• Comment: Rise of Solana, Avalanche, GameFi, metaverses.

Smaller waves / mini-seasons:

• 2020 (DeFi Summer): June — September (~90 days)

• 2019: short spikes in January and June (~10–15 days each)

• 2023–2024: there was no full-fledged alt season, only local pumps (5–10 days).