It's almost certainly OK for the 48/15 combo, but that's the wrong question. The chain needs to be long enough to loop the 48/25 combination, with about 1" to spare for the derailleur. Even if you never plan to ride crossed over, your chain needs to be long enough to do so in case you accidentally shift into it. The consequences of shifting into a combination the chain can't loop are too drastic to allow the possibility. They include a bent axle, broken dropout, bent chainrings, and destroyed RD, or any combination of the four.

Test as Hillrider suggested, but at each step pull the lower loop forward to confirm that there's extra length available for the next step.