When we had simmies, we either slaughtered them at weaning OR would put them straight on grain, and finished at 10 months. They don't do well going to grass after weaning, would frame up too much. But then, that's my opinion.
We raise Simmental / Angus crosses. We wean at 90 to 120 days, and start finishing two weeks later. We castrate at 500lbs and feed to 1400lbs+.... depending on markets etc. You need to watch your ADG and determine COG, your cross should be very good on conversion.