Keeping the flow rate low defo helps the resin last longer. I've got 2 vessels in the van, an the RO/static tank in the garden. I purposely bought a low flow submersible pump for filling the van. It takes about 90 minutes to fill 400 litres in the evening,... but even with a bad RO and low pressure giving me 40ppm before DI, I haven't changed the resin in over 6 months,...