RISC-V behind but catching up
The initial version of the RISC-V ISA with just basic integer and IEEE floating point instructions was ratified (frozen, formally published) in July 2019, almost eight years after the ARMv8-A spec was published (and which Arm had presumably been working on in secret for some years -- I suspect from around the time amd64 entered the market in 2003).
Ratification was the starting gun for serious RISC-V players.
A handful of chips existed before that -- a few made by students at Berkeley, generally with instruction sets quite different to the final version, the FE310 and FU540 made in quantities of a few hundred ("shuttle run") by some of those Berkeley students at their SiFive startup, and one commercial microcontroller, the Kendryte K210 made by Chinese company Canaan which specialises in BItcoin mining ASICs. Canaan checked out the then current snapshot of Berkeley's "Rocket" RISC-V core and built an SoC around it.
A couple of reasonably high performance RISC-V cores, the SiFive U74 and the THead C910 were announced right around RISC-V ratification in 2019. The C910 is similar to Arm's A72, as found in e.g. the Raspberry Pi 4. The U74 is similar to ARM's A55, simpler cores that can approach A72/C910 performance (and significantly better than A53) but with much lower silicon area and energy usage.
Multiple low cost RISC-V SBCs have come out this year using the U74 (JH7110 SoC) and C910 (TH1520), plus there is a 64 core C910 SoC (SG2042) that will be on boards shipping to retail customers before the end of the year (vendors have had EVBs since March).
So that's basically four years behind the Raspberry Pi 4, both for announcement of the cores (2015 vs 2019) and SBCs shipping (2019 vs 2023).
The Milk-V Mars (JH7110) is available starting from $39, with a Pi CM4-compatible board starting from $34. The Milk-V Meles (TH1520) is faster and starts from $99. Other companies including Sipeed, Pine64, and BeagleBoard are also shipping SBCs (and a tablet from Pine64) using one or the other of these chips, but so far Milk-V have the keenest pricing.
2024 is going to see RISC-V boards competitive with the RK3588 Arm A76 boards such as Radxa Rock 5 and Orange Pi 5 [Plus]. Those will be about two years behind the similar Arm boards.
Multiple companies ranging from SiFive to MIPS to Jim Keller's Tenstorrent to Ventana have RISC-V cores formally announced (or very close to it) that are in the Neoverse to Apple M1 range.