First off, welcome to the CT forum.... always glad to have a new member that wants to learn the cattle business even if it is just for having your own beef to go in the freezer...
I would start letting them out now, so that they will come in wanting their feed, and they will be accustomed to coming from "out there" to your voice calling them. Cut their bottles now, to once a day for maybe 2 weeks, then cut it out if they are eating grain good. They ought to do fine at 12+ weeks. I will bottle feed for 8-12 weeks on average... but they have to be eating good and it sounds like yours are doing very good.
Most bottle calves will keep coming back to "home" when you call them, and it makes daily checking on them easy. By the time grass gets here, they will be great to go on grass.
One thing... make sure if they get no other vaccines, you give them a 7 or 8 way black leg vaccine. Give a shot now, then in another 4-8 weeks (check bottle). Clostridiums will kill the calf quicker than anything... they exist in the environment and proliferate in some wetter conditions... they will go down and be dead before you know they are sick... It is a killed virus vaccine, cheap... and will save their lives. You can buy a small bottle at most animal/feed supply stores... just make sure it is 7 or 8 way... If they are not castrated, do that and you can use a 7way plus tetanus which would give them any extra protection when castrating... at that age castrating is usually not a problem. But we give a tetanus combo ... Covexin 8 and Calvary 9 are 2 that are combination vaccines.
It is cheap insurance against a possible loss of a calf.... Don't skimp on that if you never give them anything else...
I am assuming these will be for beef for your freezer?