18 feet, or 6 meters, though if you are hitting rock after 1m you have high soil resistivity, and thus will need to have a good number of smaller rods, spread out into an array where each one is separated from it's neighbour by it's own length, and connected to each of it's neighbours by a cable of at least 10mm, so use the 6mm cable doubled up.
Also depends if you are on top of a hill, a higher strike risk, or down appreciably into a valley, or if there is high ground near you. Going to guess you are in the northern parts of the UK, where the ground is a layer of soil over granite.
Panels i would suggest having at least 4 short grounding rods per panel, hot dip galvanised steel rod hammered in till bedrock, one per corner, and used to space the panels off the ground a little. Bolt direct to panels, so at least you can drain charge off of the panels so it does not build up. Pile of rock salt applied to the area directly around the rod to improve soil resistance, though it will corrode the rod faster, and also nothing will grow around it, so dig down a depth, then fill the hole partly, and put the soil back on top.
Still not going to protect from a direct hit, or even a hit within 200m, but at least will limit damage. To provide more protection will require the installation of a properly specified lighting system at a distance, using wide flat strip conductors to connect to a big ground mesh array, seeing as you have poor soil conductivity. Gets very expensive very fast, but again depends on location.