Hey Saman, it highly depends on the columns you use and components you want to clean up. In many cases you have to test it specifically for your compounds of interest. I found that florisil columns function by application of gravity flow so I suggest that a 1 drop per second is OK. But as I mentioned you have to specifically determine it for your compounds especially when using a vacuum manifold for application of many spe columns in a single run.
So first look in the datasheet of the spe your using. In many cases there is an information which flow is a good compromise for the applied spe cartridge. Best wishes and good look