What does Immediately mean?

[Gyanguru]

My friends' leave to remain application as Tier 4 general Dependent was rejected. The letter instructs her to either appeal the decision with in 10 business days or leave the country.
She does not want to appeal.
However, that term "immediately" is confusing? Is it hours days weeks?
Many immigration forums say one got to leave the country within 28 days but without source !!
Can anyone help please !
Thanks

[MPH80]

Under rule 320 7b - there is a concession that someone who leaves at their own expense and overstays less than 90 days will not receive a ban.

The 28 day clause can be found in Appendix FM (E-LTRP.2.2.) which allows for someone who is applying for leave to remain to have up to 28 days of overstay disregarded.

E.g. it means that if someone who is an overstayer puts in an application as a spouse or parent - providing it was within 28 days of their last refusal or appeal being dismissed then it will be considered an application within the rules.