for the dangerousness provisions of the CJA 2003 to kick in it has to be either a specified offence listed in sch 15 - specified violent offences or sexual offences i.e. s18 GBH with intent rather than s20 malicious wounding etc or a serious offence whcih are those punishable in case of person over 18 for imprisonment for life or imprisonment for a determinate period over 10 years. there are also a number of criteria that the Court will look at to see if an IPP is relevant - there must be a significant risk to the public (which I would doubt for a snatch - strreet?- robbery) and the Court if it is considering it must look at probation and medical reports etc. they will look at previous etc. client of mine with broadly similar got 21 months but as you don't say much at all about offence it is impossible to say. could be 3/4/5 years depending on circumstances. doubt IPP relevant.
sounds like it is a question for the lawyers representing the person concerned - they would be ina much better position to judge as know all the facts.