So if you pile earth over spears to whiten them, you should make allowance for at least a six-inch growth before their tips break the surface. That's when you slip a knife underneath the mound and cut the spears at ground level. After cutting the spears, let the residual earth of the mounds just crumble onto the ground. Meanwhile, the spears that you're keeping green are making the same growth unshielded from the sunlight and are also ready to harvest. This is true, but the same principle applies to ornamental bushes or fruit trees, which may take even longer.
Most vegetables are annuals.
From sowing to harvesting, they do their thing in one season. But asparagus and rhubarb are perennials. Plant them once and, after they're established, they'll produce year after year. You can grow old harvesting asparagus that you planted when you were young. Back in my 40's, I planted 16 roots or crowns.