3 hours with the CFI will not prevent him doing your GST under NPPL rules, however your terminology is curious. FIs cannot do GSTs, only examiners (FEs). If an FI happens to be an examiner as well then fine, but you may be thinking that all FIs can do your test. Not so.
Cut-off times: The GST shall be taken within 6 months of the completion of training and all sections completed within 6 months of the first attempt. For full details look at
www.nppl.uk.com .