Its really up to you as it is your tank i like about 1-2 inches of depth personally.... deeper sand beds are used to try and provide aerobic zones for bacteria to process nitrates into nitrogen gas... however sand beds are seen as nitrogen sinks, i keep a diamond goby in my 120 and he turns over all the sand very well several times a day keeping it cleaner.. a lot of approaches to this haha
You can do a DSB (deep sand bed) for a reason but you have to do it right.
It's much easier to go just as above mentioned 1 to 2 inches get a CUC that moves the sand for you over time.
You can also go bare bottom.......... new trend in the hobby.
If you ask me there's a reason why there is a sand bed on the reef