few steps:
1) add new bool variable
2) then catch start round event and start timer( set_task and start your function for specific time

+ set variable to true. ( to start menu for certain time )
3) then your function will set variable to false...
PHP Code:
public cmdWMenu(id)
{
if(variable) {
if(get_pcvar_num(gCvarMenuOpen) == 1)
return PLUGIN_HANDLED
if(get_pcvar_num(gCvarMenuOpen) == 0)
{
set_task(TASK_MENU_DISLPAY_TIME, "choose_menu");
}
}
else {
//client_print or etc...
}
return PLUGIN_CONTINUE
}
im too drunk to write full code

sry next time