In Jainism, the “cycle of Tirthankaras” usually means the time‑cycle in which 24 Tirthankaras appear.

• Time is eternal and moves in repeating cycles (kāl‑chakra).
• Each full cycle has two halves:

1. Utsarpiṇī – ascending half 2. Avasarpiṇī – descending half

• In every half‑cycle, exactly 24 Tirthankaras appear.

So: - 24 Tirthankaras in one Utsarpiṇī - 24 Tirthankaras in one Avasarpiṇī

How long is one such half‑cycle? Scriptures describe it in very huge units, not normal years:

• One full cycle ≈ 20 koti‑koti sāgaropama
• So one half‑cycle (the “cycle of 24 Tirthankaras”) = 10 koti‑koti sāgaropama

A sāgaropama itself is an unimaginably long period, so the idea is: this cycle is extremely, almost incomprehensibly long – far beyond human time‑scale.

At present, we are in the Avasarpiṇī (descending) half‑cycle, and Mahāvīr Swami is the 24th and last Tirthankara of this half‑cycle.

You can read a short explanation about the “cycle of 24” here