Was advised by neurologist to take hydrocodone if Imitrex (sumatriptan) didn't work. Can that help w migraine pain?  after 100 mg Imitrex (sumatriptan) how long till nxt pill?

Yes, it can, but... Hydrocodones is a codeine/narcotic derivatives which can relieve any pain for several hours. If used too frequently in some people who are addictive-prone, that is major problem. Presuming neurologist worked up for causes of migraine, the treatment depends on the severity and the frequency. If they occur every month, rx should try a med to prevent them entirely. Max dose over 24 hrs = 100 mg more.
Caution. Use of opiates can also lead to rebound headaches - I would minimize their use. Imitrex (sumatriptan) should be used at first glimmer of onset to nip it in the bud.
Hm.... If it is migraine headache, I would not recur to hydrocodone if Imitrex fails. Another triptan, such as Relpax (eletriptan) or zomig may be tried then. I've had good response in patients taking Fioricet despite the fact that literature doesn't want to admit to its efficacy. If you take hydrocodone on a rare occasion, I may agree to that, but for frequent migraines it is not a viable option leading to addiction.