In this context, long is an adverb and standing the present participle form of the verb stand. Together they form a compound adjective that modifies critic. Compound adjectives of the type adverb + present participle take a hyphen.
Therefore: a long-standing critic.
Source
‘No longer a chum – Pedigree withdraws Crufts sponsorship after inbreeding row’, The Times, 25 October 2008
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