This sentence is a shorthand way of saying 'Pray about IT as much as you think about it' (i.e. pray about whatever it is as much as you think about it). In which case, 'as much as' is saying 'as frequently', 'as many times', 'for the same length of time', as you think about it.

It means you can think about it as long as you want, but this is a difficult or challenging situation so, to be honest, you might as well pray. That is just as likely as praying to help.

In this sentence, "as much as" means "to the same extent" or "equally". So, "Pray about as much as you think about it" means: "Pray about it just as often or as much as you spend time thinking about it." It encourages balancing the time spent thinking with praying.

It means: when the thought comes to your mind and you are thinking about it, then pray about it.