I am a salaried employee in the state of Illinois that is not eligible for overtime pay. Due to the nature of my work, we often get last minute projects that require us to work late, and it is quite common in the industry.
Recently, I was working on a team where the other employee who was at my level had quit. My company did not hire a replacement, and I had to take on their workload as well as my own.
During a two month period, I was required to work extremely hard with this additional responsibility, including a "36 hour workday" where I worked all day, all night, and the entire next day without sleep. During this time I also worked 13 days in a row, minimum 14 hour days, without a break. Even though I had to work all weekend, I did not receive compensation in terms of additional pay, or compensated days off.
Is this legal? Does it matter that I was "fillling in" for an employee that they did not hire?
If they ask me to work these intense hours in the future without compensation do I need to comply?
Any advice would be extremely helpful.
Thank you.

If you are correctly classified as exempt, there are no circumstances whatsoever in which your employer is required to provide you with additional compensation, no matter how many hours you work or how many additional responsibilities you take on. It's not that you are not being compensated for this time; it's that as an exempt employee you are paid on the basis of fulfilling the responsibilites that your employer has asked you to complete, NOT on how many hours you work. Your regular salary covers ALL the time you work and ALL the duties you perform.

Comp time is legal for exempts but not required.

If your employer asks these hours of you again in future and you refuse, you can expect to be fired. It will be legal and I strongly suspect that you will not be eligible for unemployment in these circumstances.

I am not saying I agree with an employer asking this kind of a committment from their employees without offering something in return, but it is not illegal.

So just to confirm, is it legal to have an employee work a "36 hour" workday (from Tuesday 9 am until Wednesday 5pm)?
Are there any legal issues in requiring an employee to work minimum 14 hour days for 13 days in a row?
Are there any required hourly or daily breaks required by law for salaried employees?

So just to confirm, is it legal to have an employee work a "36 hour" workday (from Tuesday 9 am until Wednesday 5pm)? Yes.

Are there any legal issues in requiring an employee to work minimum 14 hour days for 13 days in a row? Maybe. Illinois does have a State law that requires "one day of rest in seven." There are exceptions though. You may wish to contact IL's DOL and discuss this.

Are there any required hourly or daily breaks required by law for salaried employees? Any laws that require breaks apply equally to exempt and non-exempt employees. Federal law does not require that breaks be provided and a search of IL's DOL web site (one of the more "challenging" State sites to find any wage and hour regulations at) did not reveal any laws that require rest or meal breaks be provided. If another poster knows differently regarding IL reg's, I'm sure they'll tell us.

Unless this is a truly extraordinary situation, your employer requiring you to work these kinds of hours is very foolish since it's likely to result in your quitting for a job elsewhere but no laws were broken.

In Illinois, an employee who works a 7.5 hour shift or longer is entitled to a 20 minute meal break no later than five hours into the work period.