It really depends on how much data your device is using and how often you're connecting. If you have a VPN between your data centers and the core network, the idle timeout could be as much as 14 hours, meaning the session will last for 14 hours if you don't pass any data. As long as you're passing data every 13 hours and 59 minutes, the session remains open and you don't need to send an SMS wakeup.
If you were sending a large amount of data (multiple MBs) every 24 hours, it might make more sense to just send a keep alive data packet 12 hours after you sent the large update to keep the session open. The cost of the keep alive (approx 1kb) would be negligible compared to the large update, and then you can avoid using SMS and know that your device is always in session.
If you need additional guidance, please provide more information about your expected data usage and how often you want the device to connect and send an update.
A couple of points that you need to understand around SMS and keeping "the network from timing out".
Sending an SMS message to a device will not keep an active PDP context (aka data/GPRS/EDGE session) from timing out. SMS utilizes a completely different path through the network then IP traffic. The idle timing of a PDP context is controlled by a network element called the GGSN on the mobile carriers core network. IP traffic traverses this network element, and the GGSN is looking at this type of traffic when calculating idle timeouts. SMS on the other hand traverses a network element called the SMSC, which does not have anything to do with idle timeouts on the GGSN
So long and short, sending an SMS message will not keep a PDP context alive.
What will keep it alive is sending IP traffic over the network and the length of time that the context stays open will vary greatly by carrier and is often times configurable / negotiable with the carrier you are using
As far as using SMS goes, a very common design we see in M2M is people sending a blank SMS message over the network to a device that is registered on the GSM network but does not have an active PDP context. Receipt of this SMS message triggers logic built into the device that causes it to create a PDP context to the network. The cost for sending this wake up SMS message will vary by carrier (some do not charge at all for it) and needs to weighed against any data rounding costs you might incur by dropping the PDP context.