Well 3 years at #1 and I believe another 2 years as joint #1, against several all-time great players, is pretty close to 5 years at #1. As of now, Federer had 5 years at #1, Sampras has 6. The ranking system is much different today, so who knows?
I think Groove could use the definition of number 1 of whole year just with pre open era players, when there was not a reliable ranking. Nowadays the better choice is the number of weeks. For instance, who sees Sampras 6 years as number 1 think he had more time at this position than Federer, with his 5 years, but most of us know Federer was more dominant in his 5 years than Sampras in his 6 years.