The value of the experience voucher is valid for 5 years. However, you must redeem the voucher and take the experience within 12 months from the issue date.
If you do not, and the price of the experience increases, then you must pay the increase in the price when you redeem the voucher or you can covert the voucher to credit for use towards an alternative experience.
Remember the experience must be booked and taken within the first 12 months.