Every.
Single.
Session.
But what does that mean? Well, it's easy. Every session on your plan should have a focus and a purpose that makes it worthwhile doing. That focus or purpose should be yet another step on your plan towards your overall goal. You should be able to look at every single session and understand why you are doing it and what it is contributing to your progress.
I often see athletes go out and train at the same intensity all the time, especially runners - you guys are the worst for that! Every single session gets the same effort level and that's not going to help you to run further or faster. And that's why understanding the focus of your session is key. You need to know when you're working in zone 2 to improve your aerobic endurance and you need to know when you're working at threshold to improve your lactate threshold.
You may have a different type of session focus. What about technique? I always tell my athletes that if we're working on technique I want them go nice and easy, especially if it's swim technique. We need time to focus on what our bodies are doing to be able to make those adjustments and corrections to our technique. So if you're doing a technique session, what exact piece of technique are you working on? Are you supposed to be focussing on your hand entry? Because if so, making sure you do is essential. It's not the day to get distracted by whether you're kicking enough, or whether you can hit your CSS pace.
I use Training Peaks with all of my athletes and one of the best things about that is that I can make it very clear to my athletes what the focus is for every single session on their plan. And if they aren't sure, I'm always on the end of WhatsApp, because keeping your training focussed and specific to you will help you to meet your training objectives, avoid injury and smash your goals.