Why are you switching? If your answer is:
1. "Because my agent isn't getting me out/interviews," my answer is that there is no "good" time. My reasoning behind this: if your agent isn't getting you out, it may be because you're not marketable enough at the moment. If so, you will most likely have the same experience at what ever agency you land.
2."Because my agent doesn't have enough/the right connections" then research who has the right connections. Be realistic about an agent who does have the connections you want. Are you marketable enough for them to take you on? If so, then they'll be happy to meet with you at any time.
3. "Because I'm at a new level in my career and directors/producers/etc., are telling me I need a bigger agent" Again, anytime is right because when directors and producers are making calls on your behalf, agents are interested.
For more tips and articles by top entertainment industry coach, The Greenlight Coach, visit www.theGreenlightCoachBlog.com