For 32 Bits: C:\Windows\Microsoft.NET\Framework\v4.0.30319\Config\machie.config
For 64 Bits: C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Config\machine.config

Now, how to change it?

Just add the following (for 12 hours):

< system.transactions>
     < defaultSettings timeout="12:00:00" />
< /system.transactions>

You can also try to change it with reflection:

private void SetTransactionManagerField(string fieldName, object value)
{
    typeof(TransactionManager).GetField(fieldName, BindingFlags.NonPublic | BindingFlags.Static).SetValue(null, value);
}

public TransactionScope CreateTransactionScope(TimeSpan timeout)
{
    SetTransactionManagerField("_cachedMaxTimeout", true);
    SetTransactionManagerField("_maximumTimeout", timeout);
    return new TransactionScope(TransactionScopeOption.RequiresNew, timeout);
}

Also, something like this should also work if you use it in console app before running any transaction:

var machineSettings = (System.Transactions.Configuration.MachineSettingsSection)ConfigurationManager.GetSection("system.transactions/machineSettings");
var readOnly = (typeof(ConfigurationElement)).GetField("_bReadOnly", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
readOnly.SetValue(machineSettings, false);
machineSettings.MaxTimeout = TimeSpan.MaxValue;

using (var t = new TransactionScope(TransactionScopeOption.Required, new TimeSpan(12,0,0))) {
}

So many ways to change a value which should be configurable in code in the first place, however, sometimes Microsoft just forces you to google some weird stuff on the Internet. As a homework, try to find and download WinDBG.exe in 5 minutes (without downloading whole Windows SDK or anything similar having 300MB of tools when you need just one binary).