What is the shortest interval you would consider to deliver re-irradiation for a recurrent glioblastoma?

Re-irradiation (assuming infield local progression) doesn't usually get discussed until more than 6 months following initial radiation therapy, likely because any increase before that is going to be possible pseudo progression. But after that point if there is increasing contrast enhancement suspicious for local progression, then first we discuss repeat surgical resection before we talk about repeat radiation.  

However, at some point there may not be other options, and then I have repeated radiation therapy- usually 30 Gy in 10 fx or 40 Gy in 10 fx. I target the enhancement with a 0.5 to 1 cm diameter. I agree with @Dr. First Last that there are responses but they are usually brief, e.g., a few months.    

Use functional MRI and find the active tumor. Target the tumor with a 3 mm margin and deliver 5 Gy X4-5 SRS. Don't go after edema, or areas that are not active on MRI. I have used this with success, although the response is temporary and short.

The BRIOCHE trial is a randomised trial currently recruiting in the UK randomising between PCV chemotherapy and re-irradiation 35 Gy in 10# at first recurrence.

Patients must be at least 6 months post completion of primary radiation.

https://www.clinicaltrialsregister.eu/ctr-search/trial/2019-004053-91/GB