Until when can a Netflix voucher be redeemed?

I recently received a Netflix voucher that covers 3 months, but have not yet had the opportunity to redeem it.

My question: does such a voucher even have an expiry date? Can I redeem it at any time when I want or is there a limit until when it is valid?

That depends on what exactly that voucher is. If paid for them regularly, they are valid for at least three years. If it was added to the contract or the like, these generally apply between four weeks and six months. This is determined by the person who issues and gives away the vouchers.