What is the usual progression of symptoms for a parieto-occipital glioblastoma multiforme?

Depends. If the tumor is on the left, the symptoms could include confusion while right sided tumors will only presents with numbness. Both sides can give visual symptoms but most patients don't present until they either have a seizure or are confused.
Laterality. Most likely visual processing problems, but laterality (left or right brain location) will determine if there will be impaired language processing (such as difficulty naming objects etc.) or trouble with visual-spatial orientation. Later, visual field cuts may occur and patient may gradually become more somnolent and care dependent.