A 4"-6" (100-150mm) connection to a LPHV collector is the industry norm for small "hobby" thicknessers. You might get away with less with a small planer, but will likely encounter clogging & rapid fill/empty cycling issues if you do.
Remember that (unlike planers) thicknessers are usually gear-fed. This will detrimentally affect the dust collection capabilities of low volume or high pressure collection apparatus. If swarf is allowed to collect within the feed roller apparatus of thicknessers, then pressure-damage will occur on finished surfaces, and the tool may not self-feed properly. Surface planers rely on either hand pressure or accessory feed-units for timber throughput, and are less susceptible to swarf or chip impregnation damage.
If all you have is a "cheap & nasty" HPLV setup (with or without cyclone attached) then you're possibly better off without any extraction apparatus attached at all to your thickie. At least this way chips & shavings will be allowed to fly away from the cutterhead unimpeded by any restrictive chokepoints. This can be assisted by restricting individual depth of cut settings to no more than a couple of millimetres per pass on wider planks.
A standard hobby dust extraction HPLV vac. such as those from Starmix, Nilfisk, Alto, Wap, Kraenzle, or their "badge engineered" clones from Bosch, Eibenstock, Festo, Metabo, Makita et. al. can often damage both tool/s and timber rather than assist in the chip extraction process.
