Sometimes it may be possible to combine certain discount codes together, however in most circumstances only one discount code can be applied at any given time. Although there are particular deals or offers, in which some stores may allow you to add a discount code on top of another, in a seasonal sale for example. If this is valid, then this will be outlined to you in the terms and conditions.
The validity of each discount code varies, they are subject to change. Discount codes can be valid for a couple days (for example flash sales) or even months (new customer codes). However, there are some that can only be used for a few days, so keep that in mind before you use one and double check. On Widilo, depending on the code, the expiration date is specified in the terms & conditions when you click to use the discount code and add it to your order to reduce the purchase.