There is no fixed rule that defines as to how much your post length should be in terms of content or words. Some posts can be of only 200 words, while there may be others that may have more than 5000 words. It is upon you to decide the length of your post. The frequency of your posts is much more important than your post length. Some blogs have very short posts but the number of posts each day is around 10-12. While there are others who post long posts but they have only two to three posts per week. The thing to remember is that you should stick to post frequency that suits you and the topic you have chosen.
Personally, in my opinion you should stick to posts with around 200-1000 words. Occasionally you may exceed this limit if the topic demands more from you. Personally, I try to use only enough words so that I can get my point across to my audience with making it too heavily crowded with keywords. The Readers time is important and you should not waste it by writing things that are not of essence to your topic or its relevance. A minimum of 200 words is required if you are using Google AdSense on your Blog. If you make a blog that is less than 200 words, which includes lots of videos then it’s better not to use AdSense on your blog.
Nothing is more annoying for a reader than having a blog that is updated daily and then suddenly for a month no posts at all. I have stressed a point that you should decide the frequency of your posts and stick to it. If you are comfortable with updating your blog once per day / or week stick to it. Do not ignore your blog even for a single day as it may happen that you may lose some of your regular readers by doing that. This is not the way to promote your Blog.
In my opinion, regular updates of the blog is necessary. There is always something happening around us and in our niche that is important for our readers. If you cannot offer more to your audience, it means that you are not attentive enough to foresee the changes. I am writing a post every day because it suits my style of writing. When my blog grows and I have sufficient number of posts, I may decrease my posting frequency.
Updating your blog on a regular basis has numerous benefits. It encourages your audience to check out your blog daily. Google likes sites that post fresh content regularly, so Google indexes your new posts quickly as compared to other blogs with less frequent updates. Posting frequency is of prime essence even so that some blogs may hire services of other bloggers to keep the blog updated when the author of the blog is on vacation. Remember consistency always pays, if you think you can do some number of posts per week stick to it for a long time. You will see amazing things happening to your blog and yourself.