You'll probably break even within the year. Some expect several months, others claim over a year.
Assuming .004BTC per day as raw income, you'd break even in 150 days at the current difficulty, which is about 5months.
Scratch that, I've done the math... read on below.

However, people seem to expect a 20% increase in difficulty every two weeks, so income drops 20% every two weeks.
So here's a table, assuming perfect 24/7 availability (no pool downtime, no computer downtime)

2w period	BTC/day	2w Total BTC	All-time Total
1st	0.004BTC	0.056BTC	0.056BTC
2nd	0.0032BTC	0.0448BTC	0.1008BTC
3rd	0.00256BTC	0.03584BTC	0.13664BTC
4th	0.002048BTC	0.028672BTC	0.165312BTC
5th	0.0016384BTC	0.0229376BTC	0.1882496BTC
6th	0.00131072BTC	0.01835008BTC	0.20659968BTC
7th	0.001048576BTC	0.014680064BTC	0.221279744BTC
8th	0.0008388608BTC	0.0117440512BTC	0.2330237952BTC
9th	0.00067108864BTC	0.00939524096BTC	0.24241903616BTC
10th	0.000536870912BTC	0.007516192768BTC	0.249935228928BTC

So that's 140 days, and not even close to 0.6BTC.
Depending on how much electricity costs, it may not even pay for itself.
However, the price for these things is just simply affordable now, and electrical usage is near nothing. Rule of thumb here is that a 1watt device, running 24/7, will cost €2,- (or $2,50) per year in electricity. It's a little high to allow for price fluctuation.
So an Eruptor, 2,5watt each (5V x 0,5A = 2.5W), will cost €5,- per year to run. And just extrapolating the above table will tell you it will make more than it will cost in electricity for a long time to come.
I believe that electricity in the USA costs less than half what it costs here, so it'll be profitable for far longer.
When your investment has returned completely? Not within 6 months.

Ofcourse, this does not account for overhead powercosts. A PC, RaspberryPi, etc is not included.


Edit: I've extrapolated it in MS Excel. 52blocks of 2 weeks (so 2 years), will yield 0,279997442372135 BTC
After 29 two-week periods (or 58weeks, a little over a year) the 2week income will drop below 0.0001BTC  that would be less than a single cent per 2 weeks (at 1BTC = $100 exchange).
Daily income will drop below 0.0001BTC after 17blocks (or 34weeks)

Ofcourse, this is purely based on speculation that difficulty will rise 20% per two weeks and that your mining income will drop by the same amount.

Edit2: Extrapolated through to 4 years (104 2week blocks), income total will be 0,279999999976638BTC
If the difficulty only rises by 10% per 2weeks:
after 2 years, income total would be 0,55766224756596BTC, drop below 0.0001BTC/day would occur after 72weeks
after 4 years, income total would be 0,559990240917066BTC

Edit3:
Factoring in power at €5 per year and 1BTC = €100,-, it would become unprofitable after 66weeks @ 10% diff increase per 2weeks.
At 20% diff increase per 2 weeks, it would become unprofitable after 54weeks.