You have 5 years to exhaust 4 years of eligibility, absent a hardship waiver.
Think of a redshirt as a "get out of jail free card". Everyone gets one to use if necessary.
It's a great thing to have if you get injured.
The problem is, every school routinely redshirts perfectly healthy freshmen. Granted, you get an extra year in the S&C program. The trade off is that you lose your redshirt card in the event you get hurt later on.
Look at Cordy and Dowels...
Both were Class of 2014 recruits. Their 5 year windows end after 2018.
Cordy played as a true frosh in 2014, and as a soph in 2015. Since he only played 2 games in 2016, he can apply to have 2016 reclassified as a redshirt, and he'll have 2017 and 2018 left. Assuming that he doesn't get hurt again, he'll have played 4 seasons out of 5.
Dowels used his redshirt in 2014. Even though he only played 2 games in 2016, the NCAA can't do anything for him. He'll have 2017 and 2018 left. He'll end up only playing 3 seasons out of 5.
There is always an exception to the rule.
If you're forced to redshirt due to injury or illness, or circumstances beyond your control, and you later have to miss a second season due to injury or illness, the NCAA has been known to grant you a 6th year of eligibility (basically, a 2nd redshirt year).
If you took a redshirt due to a coaches decision, you're not eligible.