Adverbs of frequency typically follow the word order rule you describe. However, those which consist of more than one word (e.g. very often, now and then, once in a while) usually go at the end. An exception to this is 'hardly ever) which takes the more common position. That being said, some adverbs of frequency (e.g. sometimes, usually, occasionally) can occupy more than one position - start of sentence, before the verb, at the end). Sometimes I find it hard to sleep. I sometimes find it hard to sleep. I find it hard to sleep sometimes.

Claire gave an excellent answer.