How long to bake half of a boneless skinless chicken breast?

Planned on cooking out on the grill but rain has ruined that. I've marinated the chicken all afternoon so I don't need any advice on how to season or marinate it I just need to know how long I should bake it for. I have two chicken breasts that are cut in half so each piece is only half a breast. About how long do I need to bake it for and it be done for sure? I don't have a meat thermometer or I wouldn't be asking.