How much money do you think a company needs per year for an ideal digital marketing plan? Just an assumption.

i think the range about 10% of budget and as Riyad said it depends on the company requirements

This depends on the company revenue i prefer to make a percentage from the net profit to invest this money and increase your next year profit you can start with 3% to 7% then you can increase it

The money required for an ideal digital marketing plan depends on a lot of variables like industry, target audience(B2B or B2C), the kind of reach you wish to attain etc.
It also depends on the various campaigns you wish to run across the year and the result that you are striving to achieve.
At the same time , that is the beauty of digital marketing.
Even with a relatively smaller budget, you can create the necessary impact with digital marketing unlike traditional forms of marketing and also reach out to the exact target group.
The general industry trend is given below in the link.
(around 17% of the marketing budget is what it states) http://www.coolinfographics.com/blog/2013/4/8/digital-marketing-budgets.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+CoolInfographics+(Cool+Infographics)

Its like asking "How much should I spend on my health for the next year?" So I suggest few questions to be answered before deciding on a certain budget:
1- Why do I need to spend on digital marketing? what output should I expect? and how it may help my organization?
2- Are we into branding only? for prestigious aspects, for leads creation ? there will be a valid Measurable ROI or not?
3- What balance should I reach between digital marketing spending and SEO/SMO cost? Some companies reach a saturation point with digital marketing spending where every extra $ they spend do not create any value added, while other companies create sales, customers acquisition through every extra dollar they spend may prefer to spend to the max!.

That's the very difficult question.
I think it depends on the company requirements and needs.
No one can assume I think.
For example if a company wants only Facebook their Budget will be different from the one who need Facebook , Twitter , LinkedIn , Google Organic and Paid advertisement.

It depends on the industry you are working at, the targeted audience, and your sales target.
In most of Arab country they spend average of20% of the marketing budget on the E-marketing.