Default Transaction Timeout

You can get the (validated) default timeout from the configuration using TransactionManager.DefaultTimeout.

TransactionOptions is a struct that encapsulates timeout and isolation level. When initializing a struct using the default constructor, it will always initialize the struct members to their default values:

TransactionOptions transactionOptions = new TransactionOptions();
transactionOptions.Timeout == default(TimeSpan); // TimeSpan.Zero
transactionOptions.IsolationLevel == default(IsolationLevel); // IsolationLevel.Serializable

If you want to specify an IsolationLevel and use the default timeout:

new TransactionOptions()
{
    IsolationLevel = IsolationLevel.Serializable, // Use whatever level you require
    Timeout = TransactionManager.DefaultTimeout
};