I get this question quite a lot and I admit that I struggle to answer it in a short sentence so I thought this would make a relevant post to fill in the gaps. There are two ways to respond to this.
The short answer is, simply when the blade starts tugging. Most commonly 3-6 months is what I see more often than not but let's get more in depth.
I truly believe that edge dulling is mostly created by the end user, not so much the by the blade doing its job and cutting the whiskers off your beard although it's the other way around with razor "blades" which is not comparable. There are many aspects, although it’s very common for beginners to dull their edges quickly as it’s only normal during the learning curve of both stropping and the shaving angle, this happens to everyone, even to the highly experienced.
Here’s the detailed answer and the bigger picture. There are questions that you need to ask yourself first.
1. How often do I use the razor?
You can use your razor every day without a problem, keep using it, keep stropping before and after every shave and store it in a dry place, it’s not all that hard. By doing so you’ll be surprised how long it will serve you. Take note that the more you use it, the more likely you are to dull it by making mistakes but it's a part of the learning curve.
2. How good is my stropping technique?
The stropping technique is another learning curve, refer to my stropping thread for more info. If you get heavy handed which happens to the best of us at times, you will result in bending your edge which is the opposite of what you are trying to achieve, or even causing microchips at worse cases or rolling an edge on a strop at worst case scenario.
3. How good is my shaving technique?
Your shaving technique will never stop improving, that’s just plain fact. The blades are designed from high carbon steel to cut through the coarsest beards daily, using them for what they’re designed for is the last thing that will dull them. It's often that we get carried away and use a high "scraping" angle (45deg+) which is working against the edge not only shortening its life but can cause irritation and razor burn.
4. How well do I maintain my razor?
Again, refer to my thread on straight razor maintenance. Examples of poor maintenance is chucking your blade in the cabinet straight after the shave. If you don’t want to oil it, at least wipe it dry and give it a few laps on the strop to remove dead skin cells and moisture that can and will cause corrosion in a short matter of time which is a real edge killer.
The verdict is, the way you treat your razor will reflect the way it treats you just like anything else. You don't need to be a scientist or a magician, all it needs is some basic TLC and you will prolong the life of the edge and it will serve you well in return.
Below is an example of an edge of a razor that was sent to me a few days ago which also inspired me to create this thread. The blade has not been maintained or stored properly and that poor neglected edge has obviously seen better days, believe it or not, this was invisible to the naked eye and it was even cutting hairs, could you imagine what it would do after it had the work?
The first pic is before I carried out any work. The second pic is the finished product and how it should look after a full honing progression which was finished on a 16k followed by a Jnat refinement. I had to remove a substantial amount of steel to get past the corrosion. Mind you this is not an old and beaten antique or vintage, it’s a fairly new and expensive modern razor.