I've never seen radishes sold as transplants so I'm going to assume you planted tiny seeds. Once the seeds germinate and you get some leaves you'll need to thin them out repeatedly or they won't have any room to grow. You can plant radishes at two or three week intervals season long to always have some ready to be pulled. I like mine the size of a large grape and that is when I pull them. You can dust a little dirt from the top and get an idea of the size that way.