I don't see why not, assuming it's less than the lambda max run time (15 minutes I believe). Use a python script and an included binary of ffmpeg might work. Note: You'll need to write to the tmp space of the lambda as that's the only place that allows writing to the filesystem and then you'll need to upload it to S3 or elsewhere.

Lambda's are good at batch jobs where you might need to kick off a few of them but not have a dedicated system for it. I've used it to automate manual customer support tasks that are sporadic in requests.