Your post seems to be confusing the rules for planning permission with those for Council Tax.
If someone lives in a caravan where there is no planning permission for residential use of the land occupied by that caravan (e.g. in a friend's garden) then planning permission is not required if the period of occupation is less than 28 days (as the it is only considered to be a temporary 'change of use' of the land). Thereafter planning permission would (in most circumstances) be required.
However the '28 day rule', above, has nothing to do with the occupation of caravans on holiday parks (which have planning permission anyway) for the purposes of deciding whether Council Tax should be charged.
In order to be subject to Council Tax, property must conform with the statutory definition of a dwelling contained in Section 3 of the Local Government Finance Act 1992 (LGFA 1992). That requires that the property must be both 'a hereditament' and 'a domestic property'.
Whether or not a caravan pitch (and/or the caravan on it) should be classed as a 'domestic property' is defined by the provisions of the Rating (Caravan and Boats) Act 1996, which requires that a caravan should be the
sole or main residence of an individual if it is to be considered a 'domestic property'. Otherwise it isn't.
Therefore, as long as the caravan doesn't become your sole or main residence, the period of time that you spend there without leaving it is entirely irrelevant as to whether Council Tax becomes payable. It doesn't.
If you'd rather read my summary in a much longer form (but with the backing of the Valuation Office Agency behind it), see here:
http://www.voa.gov.uk/corporate/Publications/Manuals/CouncilTaxManual/council_tax_man_pn/p-ct-man-pn7.html