What ranking do you have in mind? I do not tweet unless I have something important to say (frequency ~ 1 tweet/month). Recently I only retweet others.
If you want to use it for promotion, I recommend not tweeting too often, especially not more than once a day, unless there is a giveaway or some competition you are holding and you need to communicate, or if you are promoting yourself as a person (maybe you are your own brand like bloggers or some homemade cosmetics makers), but if you want to promote a corporation, it is better to give sporadic but good information, something useful and big! So, it all depends. If, again, you are promoting a small self-led business, you might want to tweet more often, but tweet either interesting quotes jokes, insights, tips.
1-2 tweets a day if you are just advertising. More if you are getting involved with the conversation.
Yes, twitter along with all social networking websites may help u for that. Use multiple accounts and tweet 4-5 times a day with each account.
I tweet around 10 times a day give or take. Any more than that you could be seen as a spammer maybe ^_^ And yea the links do help with ranking, Google seo includes social signals now from Twitter, FB, G+ etc.
Yes, I try to tweet every day manually but I also have separate softwares and scheduling systems set up to tweet out relevant content I find throughout the day. I try to post at key times during the day: Morning, Lunch-time, after 5PM and then in the evening as well.
I don't think there is a limit...well within reason. As long as what your posting is engaging, fun and something that people actually want to read you can't go far wrong! We held a competition last month and people were interacting an a regular basic with pretty much any comp related tweet I was posting, and I was probably tweeting around 5 times a day!
I post 124 tweets in 1 hour and not able to post more than 124. Twitter said that time, maximum 124 tweet in 1 hour. An you can maximum post 1000 tweets within 1 day (24 hours). You can see this tearms and rules in twitter.
I post 5-6 tweets daily but not mainly for promoting my site only. There are times I tweet generally about the daily & day to day deeds. But If you talk about twitter as a promotional tool, then again it's not a bad idea & it works..