By today's standards, I think anything over 100 starts would count. If you can say "the Twins' third baseman in 1991" and people know who you mean, he was a "regular."
And gotta say, I loved Gladden. I remember seeing him go up against the plexiglass in left field to grab a fly ball that actually bounced off it, then turning around and jogging toward the dugout as if it had been a clean catch. Smart play, batter out!