Multitabling advice

[Laughingboy]

Question for the multi-tablers: how long should you play singles, and what sort of ROI/ITM% should you shoot for before trying multi, do you think?

In a similar vein, if you're a successful multi-tabler at one level, I imagine it's still a good idea to single table for a while when moving up levels, but for how long, do you think? How do you tell if you're ready?

Thanks in advance.