The general consensus is, there is not realy time frame to give in days etc.. 🙂 It all depends on plant sp. and how well they astablish and grow.. If they are growing they can be trimmed and will keep on growing anyway.. If you have plants going nuts in 4 days time, you probably have very easy true aquatics or allready submersed form bog plants which the majority of aquarium plants are and lucky with the parameters in your tank..
Next to this, as said it depends on plant sp. and it's grow characteristics how it is best to trim them. Considering how you like them to grow to accentuate your scape.. If you elaborate on what plant sp. you are refering to and what you would like as end result, it would be a lot easier for members experienced with these sp, to give you a adequate answer or advice.. 🙂