Blackbird fledgling

Hello

I’m new to this forum.  Like a very recent query I have a blackbird fledgling in our cottage garden. We have two cats who we are having to keep in or they would kill it immediately, I’m sure. 

I know that it’s 15 days old and is now running around our enclosed garden and being fed by its parents. It’s tail feathers are just coming through and it does flutter up to very low branches of our rose bush. As it’s an enclosed, tiered cottage garden it won’t be able to leave our garden until it can actually fly. How long will this take?  It’s so hard keeping two angry cats in and bring dive bombed by parents if we happen to go near the fledgling.  Very frustrating all round but I want to help towards its survival

thanks,   Nicky