On Dell laptops the best time to do this is at 2-3 years (based on the sale date), but it may be something you can extend until it’s 4 years old with the caveat removal will be more difficult. Dell is special in at the OEM paste they use is horrible to remove (even with solvents like alcohol) once it’s dried out, especially on aluminum. It isn’t the end of the world if you wait, but you will find it is harder to remove then if you had just gotten it done within the first 2-3 years of the system’s life.
If you wait too long and find out the hard way, you can make it more bearable using something like Articlean to break the OEM paste down for easy removal. At this point I’ve seen this enough I usually recommend having it if you deal with anything made by Dell, since the paste is so horrible to remove regardless of it being a laptop or a desktop. Had to deal with an XPS 8500 that was never repasted (previous owner wasn’t comfortable doing it) and I bought it under the presumption it was going to be a nightmare like every other old Dell; I needed it.
For thermal paste I usually recommend Artic MX-4. On Amazon you may need to order $25 worth of other items to buy this (it changes from time to time), it is significantly better then Artic Silver 5. While iFixit does not have a guide for the heatsink you can work with the fan removal guide as a baseline since you probably also want to remove the fan for dust cleaning while you have the notebook open.