It is okay to formula feed only When you feel it’s absolutely necessary. Ideal feeding time should be around 20-25 minutes. If baby is taking longer and still crying, it could mean low milk supply. Other means to check for milk supply is the weight gain, if adequate urine, if sleeping well between feeds. So you can give formula feeds only after making the baby take direct breast feeds for 15-20 mins or so, and then see if her sleep/ urine/ weight improves.
Health Tips
Do not replace formula feeds with direct feeds during night time. This will affect your milk production. Take plenty of fluids and foods that improve milk production.