Short answer:

• In the Shwetambar tradition, Paryushan is kept for 8 days by established scriptural tradition. It is a focused period within Chaturmas for fasting, pratikraman, svadhyaya, and kshamapana, ending on the 8th day with Samvatsari.
• In the Digambar tradition, the parallel observance is 10 days and is called Das-Lakshan Parva.

A little more detail:

• Jain Agam-based practice fixed an 8‑day minimum period (Atthai) for intensified sadhana for Shwetambar householders while sādhus stay in one place during the rains. Over time this became the standard length of Paryushan.
• The 8 days enable a complete cycle of core disciplines: pratikraman, tapa, svadhyaya (e.g., Kalpa‑sutra pathan in many sanghs), and universal forgiveness on Samvatsari.
• Some teachers also explain the “eight” symbolically (e.g., cultivating eight key virtues during the festival), but the primary reason is the received tradition of an 8‑day intensive.

Related reading:

• Shwetambar 8 days and Samvatsari focus
• Why it is observed for 8 days (traditional explanations)
• Digambar 10‑day Das‑Lakshan outline