Threshold for moving to VPS or Dedicated hosting?

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http://serverfault.com – Is this a matter of personal opinion or are there unstated rules/factors for moving a site from Shared to a VPS or Dedicated hosting ? Rules/Factors being :- No. of hits I get per day ? per hour ? per minute ? Bandwidth consumed ? Disk space occupied ? Number of websites in my shared server ? The hardware on the server ? The type of language used ? Type of database used ? Quality of code. Amount of server customization. Web server used ? If there are any other factors and rules or numbers regarding this I would love to know. (HowTos)