Before you go adding any cleanser to your heating system you should really do a bit of research about the product and more importantly about your heating system.
X800 is designed for use with a power flushing machine and would normally be flushed around the system overnight only. It would then be drained out and the system filled and flushed with fresh water. Drained again and then you should add an inhibitor ... something like Sentinel X100.
If you do not have access to a flushing machine, personally I would use X400 and leave it in the system for 4 weeks before filling and draining the system a number of times with fresh water. When I did mine a few weeks ago the X400 was in the system for 3 weeks prior to being drained. I then added a connection to my central heating at it's highest point whereby it allowed me to run mains pressure fresh water through my heating system, until it ran clear.
( run your drain pipe into a white bucket so as to make it easier to identify any debris).
If your boiler happens to have an aluminium heat exchanger you shouldn't really leave X800 in the system for more than 3 to 4 hours..!
Do your research ....
http://www.sentinel-solutions.net/product/detail/central-heating/liquids/x800-jetflo-ultimate-cleaner/