It is best to say when you get there you will find out. How strong it should be is a relative question and has too many variable issues, circ pump, filter conditions etc. In order for the heater to work, first the "pressure" switch or the "flow" switch must close to allow the heater to work. This prevents "dry firing" of the heater, preventing it from being on with out proper water flow. So, what it will feel like is probably as different as spa brands are different.
Running on 110 volts you can expect to get 1.5° to 3° degrees max per hour to heat the tub. Depending on the temperature of "ground water" which changes by the season, but should be around 56°, it could take 18 to 24 hours to come up to temperature 102° to 104°.
When you are using the tub on 110 volts, chances are pretty good your heater is not producing any heat, thus any heat lost is not recovered until you shut the the high speed jets down.
This is the reason 90% of all the spas out there are set up on 240 volts.
