When our Bantams are laying, they lay almost every day. That being said, they tend to get ideas into their heads that make them stop laying for months at a time! I bought 3 Banties a year ago. Two of them started laying at about 20 weeks, then one of them developed a crop issue and hasn't layed since about last June or July. The second one went broody in mid-July and we gave her fertile eggs to hatch. She stopped laying until about 3 weeks ago (took about 6 months off!) The third one went broody in early November, but we didn't let her have eggs, and she has never layed an egg until about 3 weeks ago.
I definitely wouldn't count on them as your sole source of eggs, but when we do get those cute little eggs in the box, they are a sweet addition.