Question: An American rapper '2Pac' released his first solo album in 1991, how many years have passed until his next album was released?
SQL: SELECT ( SELECT groupYear FROM torrents WHERE artist LIKE '2Pac' AND releaseType LIKE 'album' ORDER BY groupYear LIMIT 1, 1 ) - groupYear FROM torrents WHERE artist LIKE '2Pac' AND releaseType LIKE 'album' AND groupYear = 1991