How long before dwarf hairgrass actually starts spreading in a low tech tank Beginner

Realistic timeline without CO2 is six months to a year for a filled in carpet, and that assumes decent light reaching the substrate. Eight weeks with two runners is not failure, it is roughly on schedule and just slower than every video you have watched, because those tanks are all injecting CO2 and dosing heavily.

Two things that speed it up meaningfully in low tech: trim it short every few weeks, which pushes it to spread sideways rather than up, and plant much denser than feels sensible. 3cm spacing is fine, 1.5cm halves your wait.

Check how much light is actually hitting the sand rather than the top of the tank. Hairgrass is the plant most likely to fail from a tall tank or from stems shading it. If your floaters have thickened up in those eight weeks, that alone could explain the stall.

Honest alternative if patience runs out: monte carlo or a moss tied to a flat stone will both give you the low green look faster in low tech. Hairgrass is doable but it is one of the slower carpets without gas, and there is no shame in swapping.

Mine took nine months in a low tech 15 gallon and then went from patchy to solid in about six weeks at the end. It is not linear. Once the runners connect underneath, the whole thing accelerates because it is effectively one plant sharing resources.

Also do not vacuum the substrate around it. The runners sit just under the surface and a gravel vac will rip up a month of progress in one pass. Hover the siphon above the carpet instead.