Your form has a TimerInterval property, which is a number in milliseconds.
If you set it, for example, to 1000, the Form_Timer event will fire after 1
second.
In that event procedure, you can close your splash screen and open the menu.
Create your splash form, and make it the start up form for your application.
In the form's Load event, set the form's TimerInterval property to the
appropriate value (its units are milliseconds, so you'd need to use 5000 for
a 5 second delay).
In the form's Timer event, issue a DoCmd.Close command to close the form.
In the form's Close event, issue a DoCmd.OpenForm "NextForm" to open the
menu.