how to know when starlxde ready to take screenshot

[ghost]

I want to use lxde under xvfb

Here is my bash code:

xvfb :99 &
DISPLAY=:99 startlxde &
sleep ?
import -window root screenshot.png

lxde need some time to start but I do not know how long I should wait. I want to ask how to know lxde is ready so I can take screenshot? If the time is not enough, I will get 'no session for pid'