Can't tell you how much or how often - that depends entirely on YOUR soil conditions and the weather. For the first growing season you will have to physically monitor the soil surrounding the shrub as well as the appearance of the shrub itself and water slowly and deeply as the need arises. Sorry to be so non-specific, but it is impossible to be more precise. You cannot rely on a prescribed timetable, as very sandy, freely draining soil will require much more frequent watering than a more organically enriched, moisture retentive soil. It doesn't sound like the shrub needs a lot of water but good establishment will be encouraged if the soil remains just evenly moist.

I've linked an info sheet about your shrub.

Here is a link that might be useful: Leucophyllum langmaniae

watering is an art NOT A SCIENCE.. ergo.. there are no schedules ...

water deeply and thoroughly ... thru the whole root zone.. mulch properly.. and then do NOT water again.. until your index finger.. inserted to the 2nd knuckle.. indicates the soil is dry or hot ... whether that is 2 days.. 2 weeks or 2 months.. only you can figure it out ...

as gal said.. how that all works.. in your area.. in your yard.. in your soil.. is for you to figure out .. only a fool could give you any better answer.. than your finger ...

a true gardener does not have a green thumb.. but rather.. a brown index finger from checking dampness ...

ken