Firstly, you should be clear about the goal ot study.Here, your goal is to carry a conversation in Mandarin. Therefore, you should pay more attention to skills of learning and speaking. Secondly, the main problem is how to practice speaking and learning in the most effective way. You should choose the materials which is suitable for your level. Materials should not be too hard or too easy for you. Thirdly, you should speak more with native speakers. They'd better be Chinese who know how to help you to develop your ability to negotiate meaning, which is the most important thing in developing communicative ability. As for time you should devote in learning, I think 1h/day is good.

With a book based course I would say 15-30 Minutes QUALITY study a day is OK. Important is the "A DAY" part, which is NOT equal to 2-4 hours one time a week. If you want to study more, and have also the ability to absorb it, I would rather do 2 or 3 times the 15-30 Minutes a day rather then 45-90 Minutes once a day.

The Tao and its Characteristics by Lao Tse English Translated by James Legge 老子《道德經》 英譯：理雅各 第六十四章 Chapter 64 其安易持，其未兆易謀； That which is at rest is easily kept hold of; before a thing has given indications of its presence, it is easy to take measures against it; 其脆易泮，其微易散。 that which is brittle is easily broken; that which is very small is easily dispersed. 為之於未有，治之於未亂。 Action should be taken before a thing has made its appearance; order should be secured before disorder has begun. 合抱之木，生於毫末； The tree which fills the arms grew from the tiniest sprout; 九層之台，起於累土； the tower of nine storeys rose from a small heap of earth; 千里之行，始於足下。 the journey of a thousand kilometer commenced with a single step. 為者敗之，執者失之。 He who acts with an ulterior purpose does harm; he who takes hold of a thing in the same way loses his hold. 是以聖人無為故無敗，無執故無失。 The sage does not act so, and therefore does no harm; he does not lay hold so, and therefore does not lose his bold. 民之從事，常于幾成而敗之。 But people in their conduct of affairs are constantly ruining them when they are on the eve of success. 慎終如始，則無敗事。 If they were careful at the end, as they should be at the beginning, they would not so ruin them. 是以聖人欲不欲，不貴難得之貨， Therefore the sage desires what other men do not desire, and does not prize things difficult to get; 學不學，複眾人之所過， he learns what other men do not learn, and turns back to what the multitude of men have passed by. 以輔萬物之自然而不敢為。 Thus he helps the natural development of all things, and does not dare to act with an ulterior purpose of his own.

I have found, depending on what your goals are, an hour or two a day if you have it, with your head actually in a book is about right. Any longer, I tend to go a little crazy. However you dont have to "study" to study Chinese, playing music, films, and Radio etc while you do other things, is all study. There's some good online Chinese radio stations, I just tend to have something like that playing in the background, while I do housework etc.

good question. the same question for me. the more time the better.