Hello,
I don't believe it is possible to limit the transformation to files that take X seconds to upload. What you can do is set up the video transformation to only occur for files below a certain size. You would have to adapt the video-thumbnail-context.xml(.sample) to use a TransformationOptionLimitsMap for the transformer.ffmpeg.thumbnail. Have a look at how the bean transformer.OpenOffice.mimetypeLimits is setup and associated with the transformer.OpenOffice bean in content-services-context.xml and apply this to the video transformer. Then you can limit it by specifying e.g. content.transformer.ffmpeg.thumbnail.maxSourceSizeKBytes=XX
Regards
Axel