Very few people have the right answer for this.
Even fewer have the right answer for your business, and that can be difficult.
Going into a long-winded explanation of how often you should post is usually inundated with 7 paragraphs of text so we will keep this short and sweet.
The only one who knows how many time a week you should post is you. Sorry.
That can be a frustrating answer for some, but you have to get a good feel from your audience. Posting twice a day can be great for some businesses, but for others posting more than 5 times a week can be annoying for your end consumer.
Local businesses tend to do better posting more since they have a connection to the community and people they are posting to. Whereas big businesses shouldn't post as much because no one wants to see that much stuff from them, on top of the stuff they are doing on TV, Radio and Print Ads.
3-5 posts a week is usually a safe number for a lot of businesses if you have issues with that 2 a week is about the lowest you can go - anything below that and you may not see growth.
Tack and Adjust.
That being said just find out what works for your specific Instagram page, local business marketing is difficult, but with Social Media Marketing, you can make things a bit easier.