user: Question: How many years was there a team that was part of the usisl pro league?
Columns: ['Year', 'Division', 'League', 'Reg. Season', 'Playoffs', 'Open Cup']
Types: ['real', 'real', 'text', 'text', 'text', 'text']
Sample row: [1995.0, 3.0, 'USISL Pro League', '1st, Western North', 'Sizzling Nine', 'Did not qualify']
SQL:

assistant: SELECT COUNT("Year") FROM "Table" WHERE "League" = 'usisl pro league';