Some pretty accurate advice posted and some close to accurate. From my experience (I was a cadet and upgraded to captain in Ryanair) You need two good sim checks under your belt in a row. As its there train set the training dept could take your OCC as 1 sim meaning 6 months later you do your 2nd. 800 hours could be 8 months with the way some boys fly in Ryanair. Also at discretion of the base TRE and training dept you could do your assessment flights as you approach the required hours in company provided you already met the minimum hours and ATPL etc then get straight on an upgrade course.
My point is you could do it in less than a year for sure. You would need all your ducks in a row for that. But lets be honest 18months is a quick command. Its hard to learn a new company and the way they play the game internally and that can slow you down a bit.
Best of luck.