Phenobarbital is a very long-acting barbiturate, which is potentially recreational IMO because it is a barbiturate. Something you must know, though, is that barbiturates are very dangerous. Mixing even seemingly small amounts with other CNS depressants can be fatal. With tolerance, the LD50 does not increase significantly, so you could become tolerant to a dose very near the lethal dose, which is not safe. The LD50 with barbiturates is very low to begin, there is not as much room for error when dosing. Because phenobarbital is so long acting, it is important you avoid other CNS depressants for several days, as the phenobarbital is still having its effect. Also, it takes some time to kick in to peak plasma levels, so don't re-dose hastily, or often.
If you have no tolerance to barbiturates, I'd say start at 50 - 100 mg. It is rather like alcohol in its effects, and like I've said several times long acting. It effects your entire system, and can produce ataxia on a level near what alcohol can do, so don't drive. Just take it slow, and steady, and read up on the drug, and barbiturates in general.