There is nothing inherently wrong in this pattern. I have used the daemon function in init.d to start a very similar python script. As long as "do something" doesn't leak, it should be able to run forever.

The code as you post it works just fine. The problem may (as mentioned by @Guy) be cause when the user inputs something that is not exactly an int. That is because input returns a string and int tries to get an integer out of that string e.g. in. On failing to read int raises a ValueError. e.g.

>>> num = input("Enter an integer: ")
Enter an integer: 12.5
>>> num 
'12.5'  <-- num, the return of input is a string
>>> int(num) <-- int fails to get a integer out of the string num
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '12.5'   

So you need to handle that case explicitly by a try except block

import time as time
while True:
    for i in range(2):
       try: 
           num = int(input("Enter an integer: "))
           print("The double of",num,"is",2 * num)
       except ValueError: 
           print("Please enter a valid integer")
    print('10')
    time.sleep(10)

The slow CPU wasting part is the "do serial sending". The while loop with just a short sleep will use negligible CPU.

Can you show the serial sending code. There may be a way to speed that up.

On this rather slow CPU I see this:

import time
while True: time.sleep(0.2)      # 0% CPU
while True: time.sleep(0.02)     # 0% CPU
while True: time.sleep(0.002)    # 0.5% CPU
while True: time.sleep(0.0002)   # 6% CPU
while True: time.sleep(0.00002)  # 18% CPU

Now do some extra work in the loop:

import time
while True: range(10000) and None; time.sleep(0.2)      # 1% CPU
while True: range(10000) and None; time.sleep(0.02)     # 15% CPU
while True: range(10000) and None; time.sleep(0.002)    # 60% CPU
while True: range(10000) and None; time.sleep(0.0002)   # 86% CPU

I ran those in the interpreter and stopped each while loop with ctrl-C.

Keep track of the next time you want to print the time.

def main():
    next_time = datetime.datetime.now()
    delta = datetime.timedelta(seconds=30)

    while True:
        period = datetime.datetime.now()
        
        # do something here
        
        if period >= next_time:
            # every 30 seconds do something here 1 time, like saving in database
            print(period)
            next_time += delta
            # Or, depending on how long the main body took
            # and when you want to do this again,
            # next_time = period + delta

Something like this:

while not x: time.sleep(0.1)

will wait until x is true, sleeping a tenth of a second between checks. This is usually short enough for your script to seem to react instantly (in human terms) when x becomes true. You could use 0.01 instead if this is not quick enough. In my experience, today's computers are fast enough that checking a simple condition even every hundredth of a second doesn't really make a dent in CPU usage.

Of course, x should be something that can actually change, e.g. a function call.