Lichen & bleach - How log to leave bleach on paving

[Scrubber!]

Hi - I have the usual black spot on my Indian Sandstone blocks, laid with Rompox pointing, and I note the advice to initially use a 10% bleach solution and plenty of scrubbing to deter the lichen, but that it must be washed off afterwards. But I couldn't find any info about how long you should leave the bleach before washing off, or perhaps the other way round, i.e. what is the max time to leave the bleach at this (and perhaps other stronger) solutions.
Cheers

[RAPressureWashing]

I'm going to assume you are using a pressure washer?, so give the paving a clean first to remover any loose stuff and general dirt etc, then apply your bleach mix on to the paving, dwell time is around 20-30 mins depends how bad the lichen is, after that time wash off and see what you have left, you do have to do this a couple of times sometimes as lichen can be stubborn to move.
Things to remember, bleach can kill plants and scorch grass so pre-wet if you are working close to a lawn etc, don't get it on lead as you'll turn it orange!!! plus your PPE. There is a bleach mix of mine on the main site, it's on the stain removals page I think, gives you all the details you'll need.

[Scrubber!]

Cheers Roger, Thanks for clarifying the timescale for leaving the bleach. All your points noted. I do have a pressure washer, which I will use first as you suggest, but elsewhere on the website advice about cleaning there is much emphasis given to plenty of elbow grease and a scrubbing brush, so I am planning to apply the bleach solution and scrub for the 20-30 mins you suggest (if I last that long) and then wash off. As you say I fully expect to have to do this a few times.
Thanks again
Gary

[RAPressureWashing]

Scrubbing can help with Lichen isn't going to hurt and can help, just nothing metallic, a stiff nylon bristled brush etc is fine.
Washing the stone first does a couple of things, one gets rid of all the loose easily removable stuff and two, soaks the stone so the hypo (bleach) solution sits on the surface working.