Generally mines 1500-3000 words. something like this usually:
10 very REALISTIC steps on how to make better videos on YouTube.
Whether you're just starting your YouTube channel, or have been making videos for a while, these tips will give you ideas on how to make great videos and get more subscribers, which means you'll get more views.
Always remember, views come after you get subscribers. I wish everyone watching this the best of luck in improving their YouTube channel.
Also when you realise this was all bull, theres some more bull for you in the link below.
Aim to have descriptions around 200-300 words that describe your videos. Just make sure you're writing in natural English, and at the same time aim to use as many of your tags (that you hopefully researched).
Your desc. should be as along as you want it to be. Some tips I would use: always provide links to your other social media sites, site sources/contributors, and leave a description that helps google/youtube find and rank your videos.