One hundred days
Onitsura (previously) sez:
The haiku masters of old said, "Better a single day joining in than a hundred days of practice"; attendance at haiku circles was what they deemed important. The words of substance and action for mountains, shores and dwellings; the working and fixing of phrases; the clashing and overlap of themes; the difficulties of these and other issues of criticism mean that a single day's participation can indeed be most important.
There is a lot of renga/haiku jargon in this. I'm not hip to the precise details, and would no doubt have gotten myself stabbed at a medieval renga party, but here are a few notes: "Words of substance and action" is an ad hoc translation of taiyū 体用, which literally means "body and effect", a scheme for dividing words on a certain topic. For example, when considering the ocean, "sea," "shore," and so on are "body" words, while "wave" and "roar" would be "effect" words. (This word/concept is not used in traditional waka criticism, raising the interesting question of who thought it up, and why.)
"Clashing" corresponds to sashiai 指合, also known as sarikirai 去り嫌い which refers to when too-similar words appear too close to each other (or, by extension, the rules to prevent this happening). "Overlap" corresponds to 輪廻, which means saṃsāra; in the context of renga/haiku it originally had a very precise definition which I am a bit fuzzy on but later evolved to mean ugly repetition in general (a subset of sashiai, I think).
The point of all this is that there are so many detailed rules for renga and haiku, rules dependent on context, that it becomes an intensely social activity. There is room for practice on your own, but if you can find a group to join and meet with regularly, you can save yourself 99 days. And this is quantitatively different from just having a circle of friends who also write poetry with whom you can exchange sonnets in progress and discuss the latest outrages of Lord Byron. (Although it's a bit more like the modern sort of workshop where people are encouraged to criticize everything down to the whitespace.)
I doubt I'm the first to make this analogy, but I guess you can liken renga to jazz in this respect. Alone, you can work on your technique, listen to and play along with classic records, read books on what scales and modes sound best over the diminished sixth and so on — but none of it will be that helpful unless you also find a way to get on the bandstand regularly. You can't get good at collaborating with others without, well, collaborating with others.