Thanks.No, I can't think of any way off the top of my head. You can manually change it for each schedule you create... but I don't think the default can be changed.
With "until space is needed", recordings will never be automatically deleted unless you choose to enable disk quotas (explained in the manual as well):
Recording - MediaPortal 1 - MediaPortal Wiki
...so I would have thought you should be safe with "until space needed".
I just saw while going through the server config that i dont have the disk quota option enabled anyway. So i figured it might be ok.