Spreadbet- rotation strategy-some help please?

Hi, I am new to the site and am currently experimenting with a rotation strategy using a spreadbet account.

My idea is as follows:
Run my own screen- its largely based on the Winning growth and income screen
Filter the results via a manual rule set
Arrive at a basket of shares to invest in
Open a quarterly spreadbet position on each with a guaranteed stop in case of an "event"!
Hold....

Question is- hold until when?

I take the view getting into positions is often the easy part its knowing when to exit them that is critical.

Two options seem to be:
1) Hold for 3 months (the duration of the spreadbet contract) and then weed out those that no longer qualify and replace with those that do -along similar lines to the guru screen rotation. In other words only run the screen every quarter.
2) Hold until the stock no longer features as a selection( whenever that may be) and replace and add new qualifiers as and when they appear which will likely result in a lot more movement in the positions.

There might be other ways of doing this but I thought I would ask for some help here- anyone any thoughts on this approach?

Any help would be much appreciated

Many thanks

Mark