How Much Time Should Elapse When Switching From Fentanyl Patch

how much time should elapse when switching from fentanyl patch to oxymorphone? only used two patches and didnt help pain level. was on methadone for longest time before trying the patch, drs. out here nevada wont prescribe the methadone. dont want to have reaction switching meds. patch removed 4hrs ago. can i take the oxymorphone er 20 now or wait? thanks