I spent about 3 months in odd evenings writing the first couple of episodes of my first sitcom. Sometimes I'd have bursts of activity, then nothing for weeks. Another 2-3 months afterwards rewriting before I sent off to Writersroom. They took about ten weeks to get back to me, with a polite "no".
My best advice is:
1) write two episodes first; I only got a real handle on my characters after writing the second episode.
2) invest in a professional scriptreader to do a report on your script before sending to BBC or other production outlets. Unfortunately I only did this after my Writersroom rejection, but the report I got helped me realise why I got the polite "no".