Ramp tests are planned at the start of every block in base and specialty, and also midway in build. So for the two base blocks it would be once every 6 weeks. Build blocks are 8 weeks but have a ramp test after recovery week 1. I’d say you could skip the one in the middle of build if you don’t like them. You might improve a bit in the first 4 weeks but even then you’ll still be in the right zones for the 2nd part of build.
The reason you’d want to test frequently is to keep making sure your baseline is correct. To far off and you’ll be training different energy systems than the plan wants you to do.