So taking over a program, how long will it take to change the offensive and defensive philosophy? and should I put more practice time in each till they get to where they need to be in each?

I don't know how long it will take for your players to become proficient in your new OD, but I have heard the the way to approach it is to put all of your practice time into the new O/D and continue running the old O/D until their IQ's in the new one catch up. You'll probably get emails from your AD asking why you're running an O/D that you're not practicing. Just ignore them.

As a general rule of thumb, don't put more than 25 minutes into your O/D in terms of practice time (and that's 25 minutes each, or, 50 total). Towards the end of the season in which you're changing sets their IQs will hit the C+/B- range and it's pretty safe to play the new sets at that point. Fwiw, most users put between 20-25 minutes into both a team offense and team defense.

I'm currently doing this exact thing and just finished non-conference play (10 games). My guys are mostly C or C- in the new offensive/defensive philosophies. Be prepared to take some L's. It is not an easy transition.

25m in O/D and, at the end of conference reg season, you should be B-.

Different note - unless you plan to stay for a while, consider just sticking with the existing scheme - if the school is a stepping stone toward a higher level likely better to stick with it, improve and move on