An American rapper '2Pac' released his first solo album in 1991, how many years have passed until his next album was released?
2Pac is an artist; album refers to releaseType; groupYear = 1991; SUBTRACT(groupYear = 1991, groupYear where releaseType = 'album' LIMIT 1 OFFSET 1);
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