Firstly, I would to say that this is hashcat stuff is new to me so I may be (am?) doing this wrong but it appears that unless you are using dictionary based PSK's it would seem that it may take a (very) long time to decrypt an 8 digit PSK if you get hashcat to use all of the alpha numeric and special character options.
Ps I only have access to some older GPU's (2xHD7700 and 1x GTX660) so 3 newer ones would be faster (but I don’t know by how much).
Session..........: hashcat
Status...........: Running
Hash.Type........: WPA-PMKID-PBKDF2
Hash.Target......: c1b8f6e1f057e8e25189a8f848ef7568*20a6cd88c802*94652...63726b
Time.Started.....: Thu Aug 09 19:51:03 2018 (13 secs)
Time.Estimated...: Next Big Bang (> 10 years)
Guess.Mask.......: '?a?a?a?a?a?a?a?a' [10]
Guess.Queue......: 1/1 (100.00%)
Speed.Dev.#1.....: 35342 H/s (71.94ms) @ Accel:64 Loops:32 Thr:1024 Vec:1
Speed.Dev.#2.....: 45742 H/s (55.24ms) @ Accel:128 Loops:32 Thr:256 Vec:1
Speed.Dev.#3.....: 45742 H/s (55.24ms) @ Accel:128 Loops:32 Thr:256 Vec:1
Speed.Dev.#*.....: 126.8 kH/s
Recovered........: 0/1 (0.00%) Digests, 0/1 (0.00%) Salts
Progress.........: 983040/6634204312890625 (0.00%)
Rejected.........: 0/983040 (0.00%)
Restore.Point....: 327680/6634204312890625 (0.00%)
Candidates.#1....: 'O_ !!!!!' -> '@-yginer'
Candidates.#2....: 'r0qwoner' -> 'kF+19999'
Candidates.#3....: '(ZDERINE' -> 'ux+19999'
If I set hashcat to only bruteforce using number then it is quicker.
Numbers only
Session..........: hashcat
Status...........: Running
Hash.Type........: WPA-PMKID-PBKDF2
Hash.Target......: c1b8f6e1f057e8e25189a8f848ef7568*20a6cd88c802*94652...63726b
Time.Started.....: Thu Aug 09 19:58:32 2018 (9 secs)
Time.Estimated...: Thu Aug 09 20:11:44 2018 (13 mins, 3 secs)
Guess.Mask.......: '?d?d?d?d?d?d?d?d' [10]
Guess.Queue......: 1/1 (100.00%)
Speed.Dev.#1.....: 35324 H/s (71.97ms) @ Accel:64 Loops:32 Thr:1024 Vec:1
Speed.Dev.#2.....: 45697 H/s (55.24ms) @ Accel:128 Loops:32 Thr:256 Vec:1
Speed.Dev.#3.....: 45698 H/s (55.24ms) @ Accel:128 Loops:32 Thr:256 Vec:1
Speed.Dev.#*.....: 126.7 kH/s
Recovered........: 0/1 (0.00%) Digests, 0/1 (0.00%) Salts
Progress.........: 655360/100000000 (0.66%)
Rejected.........: 0/655360 (0.00%)
Restore.Point....: 0/100000000 (0.00%)
Candidates.#1....: '23456789' -> '79561123'
Candidates.#2....: '25580456' -> '71512200'
Candidates.#3....: '21512200' -> '76865434'
Also note that all the examples I found on the web have part of the the psk in the guess mask already.
For example;
./hashcat -m 16800 test.16800 -a 3 -w 3 '?l?l?l?l?l?re!123'
As I understand it thier mask will randomise the first 6 characters and always append re!123 in their example, so I would expect thier result to be quick.
Lastly do home grade routers have 802.11r enabled by default (or even support it).