Depends on your budget, your target and niche that you chosen. You should get the income 3 fold times from your budget.
The question is whether you've optimised your layout for the Adsense to convert! go with a small budget and assess your conversions. 100 visits is a good enough sample for you to get a realistic idea as to whether your layout works.
It also depends on what keywords you are advertising for. If you're in the US the lowest bid you can submit is 1 cent, and 5 or 10 cents as minimum bids if you are in other parts of the world. So depending on what you need to pay per click. I think minimum amount to charge your Adwords account is $50 or if it was $20, don't remember. For the sake of the argument, let's say $50 is the minimum charge. Then if you find 1 cents keywords, keywords no one else is bidding for, you can get 5000 visitors for your $50! But let's say you instead have to pay 50 cents per visitor/click, then you only get 100 for your $50. A realistic amount to try things out and see if you make your money back, manage to get your visitors to like your site and return, I would say between $50 and $100 would give you a good idea if it's worth to continue or if you need other sources. Don't burn all your money at one! Do a two week test first, it's important to see the traffic changes over weekends and just have something to compare one week to the other. Then do some changes, either to your Adwords campaign or to your site and then run a new test of two weeks and then when completed, coompare the two campaigns to see which one did best and try to figure out why. Then you base your calculations on wheter or not to continue advertising via Adwords on the numbers from your best campaign.
I think you should test the waters carefully. Check that your page converts for what ever you want it to. (users click adsense, users buy your product or users buy affiliate product or what ever) You can then start to work out what it costs to "buy" a sale.
It all depends on how much your commissions are and what your conversions look like. Usually, when starting out, its best to divide your total commission by 100 to determine your break even point, set your max bid at that price, and hope you make a profit. So, if your commission is $25, you should bid no more than 25 cents per click.
I think it's a totally personalize theme.It depends upon you that how much you will spend for a particular advertisement?