There are several other ways of saying this in English: it’s been ages since I last saw you!/ it’s been a long time!/ it’s a long time since I saw you! There is a phrase doing the rounds for the Latin equivalent of this expression, viz. tam diu minime visu! but it doesn’t somehow ring true to me. What would good concise equivalent expressions be in Latin?
If you want something as concise as the English, one options is longe invisus, "unseen for a long time". If you use this as a greeting, you can add (implicitly or explicitly) es or switch to vocative. If you use this one, be sure to adapt it to the gender and number of the person or thing that hasn't been seen for a while.
This is not a fixed phrase as far as I know, but if a reunion between two characters begins with a warm greeting Marce, longe invise! most readers will have no difficulty in following the thought. The grammar is simple, with just a participle modified by an adverb. I hope there is something more idiomatic, but before such suggestions surface this is what I would go with.
This approach can easily be varied. You can change longe invisus to tam diu non visus or any other such expression. There is no fixed way to go about this, so context, taste, and potential ambiguities to be avoided need to be taken into account.