Tough question this one.
We can put up nice graphs showing how SAP implementations are now done under x months, and that upgrades to ERP 2004 are much easier than in the past. Several partners have built additional conversion tools too, especially to move stuff across from the dark side. There is a lot more experience out there. Things have sped up over the last 3-4 years, but making software easier to implement and upgrade still remains a key issue for all software vendors, SaaS or onsite.
That said, I have a different take on that question.
The best answer to this question is with another question.
"How long did it take you to buy it ?"
Organisations that can make procurement decisions quickly, 99 times out of 100 can implement quickly. If you take years to decide whether to buy or not, it is likely that the implementation process will looooooooooong too. The biggest factor in software implementation timing is the organisation's ability to make decisions and follow through on them.
I know of two companies, I won't name them here (yes chicken). Same size ,same industry, same geographies, same scope, same start date. One went live with 50,000 employees in 50 something countries in less than a year, the other was still in pilot 30 months on.