An IPP is an Indeterminate Sentence for Public Protection, when the offender is considered to be a real danger to to the public.
This means that your father has no right to automatic release at the end of his sentence - the parole board must be satisfied he is no longer a danger.
Your dad could not have 'smacked some bloke' - it must have been a very serious assault.
Before 14th July 2008 the sentence for IPP was minimum 10 years, it is now minimum 2 years.
Without knowing the precise sentence your dad got, it is impossible to answer.
With an IPP, however, he could be kept in much longer if he is considered to still be a danger to the public.